Ishibumi did say you can read Slash Dog first, lol. But yeah, I think it's better to read DxD first.
Feats and statements? I remember some of them for sure, though I might not have a perfect recollection. Not that Low 6-B Maou-class statement stuff, I think that's retconned.
Low / Mid-class
There's not really anything for people with this level of power. However, if we count the Volume 1 Slash Dog protags, I think they'd be at this level of power, with the exception of BxB Tobio and Lavinia of course. So:
- Kouki piercing through the floor of a building.
- Jin destroying multiple three meter tall clay dolls, whose arms alone are nearly as thick as telephone poles.
- Natsume's Griffon mangaged to cut off the arm of one of the aforementioned clay dolls. The evolved Griffon can casually destroy them, blew off the roof of a factory and destroyed a bunch of cultivation tanks.
- Kouki's evolved Byakusa destroyed half of the body of one and fried another with its electricity from the inside out.
High-class
- Volume 2 Issei putting a large hole in a mountain. Rias described it to be the firepower of a High-class devil. Later on, even though he was holding back his power so that he wouldn't ruin Rias' strategy, he completely destroyed a tennis court and the other sports grounds in the vicinity.
- A drunk Rose destroying a town.
- Volume 9 Xenovia's feat done with Durandal Cannon. She destroyed...I'm not sure what she destroyed, but we don't get to see the full size of the crater, and Xenovia said she adjusted the power of the attack so it's likely higher.
- BxB Issei easily destroying a mountain without meaning to, to the extent that it's described to have vanished entirely. I wouldn't scale him to normal High-class devils because Issei's shown to be a particularly strong one.
- BxB Issei splitting a sea with Dragon Shot. No specifications, unfortunately.
Also, Akeno and Kuisha were destroying large towers with their normal magic and Akeno's lightning destroyed a host of them. Georg created a replica of Kyoto, but I'm not sure where to place him as he blocked Xenovia's aforementioned Durandal Cannon.
(Akeno also destroyed a gymnasium in V2, though she couldn't use that attack repeatedly)
Ultimate-class
- Star Buster's Pawn lifting and throwing small mountains / mountaintops.
- Star Buster's Bishop burning the surface of an entire island.
- Ajuka's servant creating a dimension with an island, lakes, and mountains.
- Galvaldan can destroy a mountain according to Azazel.
- Triana Issei destroying the city despite the dimension being constructed to be particularly sturdy. It might be higher since Issei unusually mentioned vaporization and it did not sound like a boast iirc.
There are such things like BxB Sairaorg and CxC Issei shaking the Rating Game field, which is Agreas (described as a large / gigantic floating island). Though I don't know if they shook the whole thing.
Maou-class
- Ruma Idra can tear off mountains.
- (Boosted) CxC Issei destroying a small mountain with the shock wave of his punch.
- Xenovia destroying a large portion of a large island. I don't know if current Xenovia is Maou-class without Cross x Crisis though...
- Juggernaut Drive users destroying mountains and islands.
- Serafall can bust mountains with ease and is stated to be capable of destroying an entire island with a single shot of her demonic energy if she were to get serious.
- The first generation Maou using their power to levitate Agreas. Their tech is there so they might have used that to levitate it, IDK.
- The statement that Dulio can manipulate the weather of a country. It's not similar to the Low 6-B statement for Maou-class in the earlier volumes; it's stated several times that Longinus can work on a country wide scale. Though it's nerfed by recent storm revisions.
God-class (Combat Type)
- Star Buster easily erasing several islands with his rifle without Balance Breaker, which would be a pretty significant powerup considering Star Buster Star Blaster is geared entirely towards raw power. His sword can cause an entire island to shake.
- Trihexa's split bodies can one-shot mountains, islands, lakes, and forests with their fire breaths.
- Nyx easily destroying Rose's barrier that spanned an entire island.
(Lower) Transcendental-class
- Rizevim could have destroyed Agreas.
Top Te
- Vali casually destroying a 5000m tall mountain. It was stated to have "disappeared instantly" in the Japanese raws.
- Vali's Satan Lucifer Smasher tearing through the mountains in a mountain range, with the attack travelling to the edge of the horizon. It turned the area into a barren wasteland.
- Pseudo DxD Issei's Infinity Blaster annihilating the majority of a mountainous terrain filled with towering mountains, valleys, rivers, and trees. The area is described to be completely barren after the attack.
- Pseudo DxD Issei and Ddraig's attack dwarfing an entire island (got it from a spoiler though).
- Azi Dahaka's magic dwarfing the sky above a mountain range.
- Ddraig hollowing out a portion of the fake Yggdrasil, which is described to be big enough to pierce through the clouds.
- Ddraig's fire breath covering the entire sky of a Rating Game field that contained several large islands.
- Crom's attack creating a colossal hole both in the middle of the sea and also the sky.
- Typhon easily changing the weather of a large island in a single attack. His control over the weather is a few levels above Dulio's, who can manipulate the weather of a country.
**** those storm revisions.
- Sirzechs destroying half of an island while holding back his power.
- Sirzechs' transformation shaking the Realm of the Dead and cracking its walls even though they were constructed to be particularly durable. Unfortunately, its size is unknown.
- Scale Mail Loki's attack would have caused the world map to be carved and repainted.
- Counter Balance Azazel bisects the same attack with a simple swing of his trident, with the resulting shock wave bisecting the ocean. Note that there was a limiter on Azazel's armour.
- Simple sweeps from Scale Mail Loki and Counter Balance Azazel were splitting the sea, grinding mountains a distance away, and freezing the surface of the ocean.
Unknowns
I'll categorize them like this because of some uncertainties.
- Lavinia freezing a small country with Absolute Demise at her maximum capacity. Dunno if this is referring to the normal ice princess or her Balance Breaker state, but I'd say I don't see Lavinia being God-class for now. It's not a hyperbole because it was stated in a normal way by the narration, and it's consistently stated that some other Longinus can work on the scale of countries. True Longinus, Zenith Tempest, Annihilation Maker, Dimension Lost.
- In Slash Dog 3, it's stated that three legendary youkai can destroy Japan quickly if they're unsealed. The spoilers say "in an instant", but hey, let's wait for the actual translation.
- Neried Kyrie manipulating the sea / ocean.
- True Longinus being stated to destroy a country. How it would do this is currently unknown. Maybe with a super large blast of light?
- Annihilation Maker destroying a small country in "mere hours". Azazel said Boosted Gear and Divine Dividing are ahead in raw power, so I would assume Prime Ddraig and Albion can do better.
- Ajuka creating a dimension the size of the Australian continent. Timeframe and all that jazz is unknown.
Dragon God-class
Well, you know how this is. Planet destruction hype but no concrete feats. I'd attribute this to Ishibumi's apparent reluctance to show country level damage, since they say destroying a country would change the balance of the whole world and gods also don't want this because they exist as a result of humans.
Melvazoa, Regalzerva, and Seraselbes also have the Earth destruction statement, as seen here ----->
https://jerutz.wordpress.com/high-school-dxd-ex/top-secret/. Never mind that if you know about them already.
These are all I can remember, currently.