For starters, there's
these videos.
Yakone as seen in the 2nd one especially, was able to prove more than a match for even the council of Republic City at the time thanks to his Blood bending. This includes, Adult versions of
Toph Beifong,
Katara, and even gave
Aang some trouble. Aang even needed to go into Avatar State in order to defeat him. After losing his bending and escaping his prison, he trains his sons Noatak/
Amo and Tarrlok to become bloodbenders. Amon was also thought to be stronger than his father ever was. But both Amon and his brother are both able to use Bloodbending without the need for a full moon, something Katara has never done. I'm unsure about comparing him to Sozen's comet Ozai, but At least
High 8-C seems reasonable for Amon. Tarrlok should be comparable if not superior to Katara, and there's also
Tenzi and
Lin Beifong who should be pretty comparable to Tarrlok and Base
Korra.
After the first Season of Korra is where it might get complicated.
Unalaq for sure should be stronger, his Avatar State form is legit where it's at, but his Base form should definitely be upgraded as he's superior to Amon and base Korra for sure. The third Season also introduced
Zaheer,
Ghaza,
Ming-Hua, and
P'Li. P'Li is quite similar to
Combustion Ma, so he could scale to her, being comparable to her allies and a threat to Base Korra sounds like a better justification. There was also the video Cal linked above, if someone's willing to calc some of the lava bending feats, that might lead to some potential upgrades for Season 3 Korra characters and beyond, but for now, At least
High 8-C is good for them all.
Boli did pretty good against Ghazan, and
Mako shouldn't be too much weaker.
Suyin Beifong might also be comparable if not stronger than her sister Lin, so she could be
High 8-C as well.
Last but not the least is
Kuvira. She's probably the strongest Metal Bender in the series, and might be on the level of Zaheer and the others. She's definitely At least
High 8-C at bare minimum. And with the
Colossus, she should be even higher. The cannon is
7-B for sure, but I'm unsure about the durability or striking strength.
I'll wait for Dark649 to come back and see if there's anything I missed.