Speaking of Issho's feat, I believe it will definitely be one of the most significant topics to discuss regarding the scaling. It isn't only applicable to the top tiers (though it likely won't change much, since most of them are already in Low 5-B), but also to virtually every major fighter in the New World, as they either directly or indirectly downscale from, scale to, or upscale above that feat.
The primary examples would be Jozu, Vista, Ace, and Marco (as yes i already have him at possibly Low 5-B, its baseline would still be 5-C). As during the Marineford Arc, they were the only Whitebeard Commanders shown to be powerful enough to match and withstand casual/unnamed attacks from Top Tiers (which also would include Jinbe due of performing similar feats in Marineford and later arcs).
From
Jozu be able to block a casual slash from Mihawk and make Kuzan bleed and stalling him,
Marco be able to briefly clash with Borsalino or kick Kuzan away,
Vista be able to block and stalling Mihawk for a while,
or a tired Ace be able to nullify Kuzan's Pheasant Beak (thought its also due of the elemental advantage).
Attacks which can be assumed to be at least comparable to Issho summoning and dropping meteors with little effort, logic that can be apply to any casual/non named attacks from other Top Tiers like the Emperors, Advance Conqueror Haki Users or any characters who its legit potrayed as be on their level.
And if we accept this, then the scaling chain would not stop as this. As back in Marineford, Doflamingo's strings
were durable enough to restrain Jozu with no indication that he could have free himself. The strings be that durable its also supported later in Dressrosa, as his Birdcage (which its made by the same strings)
was able to withstood the same Moon level meteors, which again its a feat that prior of that only characters on the caliber of WB's Top Commander ever done it before.
Not to mention that
nearly nobody was strong enough to either damage the Birdcage or even slow it down alone (minus Issho who was purposely let it happen while Luffy was busy with fighting Doffy), including
Kin'emon, who should be more or less around Jack's level, further proving that Doflamingo's Ito Ito no Mi should have at least similar if not greater toughness than most Top Commanders, even on the caliber of Jozu, putting the strings at 5-C.
Which then will apply to Boundman Luffy who was show to be strong enough to
shatter the same strings by just transforming, and this doesn't take into account the
tensile force from the compression effect, which (if i'm not wrong) its currently accepted as a 3 times Multiplier, meaning Boundman's AP/Striking Strength can go up to 9.489 times above baseline 5-C, which would be
Moon level+.
Which then would also affect Doflamingo's physical stats, as while much weaker than Gear 4th Luffy, he was still able to survive multiple named attacks to the point that Luffy run out of stamina and needed to use his strongest move to trully KO him, and while Doffy having insane endurance does help him its also unlikely he would have resistance for that long if he didn't have the necessary Durability, meaning we can assume that Doffy should at least scale at baseline 5-C, while only with strings would upscale above that to the 3.163 above baseline from Jozu (only in terms of Durability and Lifting Strength as Doffy never once manage to trully damage Boundman or Jozu).
This would also affect his regular AP/Striking Strength as in general excluding specific attacks or outliters such as the Lunarians, Durability should scale to AP/Striking Strength and viceversa, which its show while Doffy be stronger than Luffy (Pre Gear 4th) and Law, who were still capable to injure/bleed him despite the gap in strength. Also Doflamingo was able to
injure Kin'emon with a kick while distracted, who should <= Jack the latter who also his own scaling that put him above baseline 5-C, and was potrayed as a threat (not necessarely stronger than him, but nothing on the scale of Boundman).
From Jack in general potrayed as stronger than Doflamingo, from scaling from the other Red Scabbards which combine might of 4 of them can make Dragon Kaido bleed, and because in general even the weakest of Kaido's All Stars should at least downscale from WB's Top Commanders such as Jozu, due of the Emperors and their crew be able to stalemate each other for years, same can be apply to Charlotte Cracker (True Self not his Biscuit Soldiers) as he was even stated to have Armament Haki too strong for Luffy (something that he never say against Doflamingo), with his Pretzel having even higher AP as it could actually harm Boundman Luffy.
Thought in the case of Dressrosa Luffy (from Base to Gear 3rd) and Law i don't believe they should get to be fully 5-C and rather similar to Marco they should only get a possibly 5-C, with their baseline be 6-A
from downscaling from string's bare minimun durability, or if that doesn't work then the 6-B from Dressrosa Zoro cutting Pica in half can work too.
