BB wasn't afraid of fighting Garp and Sengoku two on one doesn't mean he would have won. Plus if Kuzan was ever to turn on BB if wouldn't just be BB vs Kuzan, it would be the rest of the BB pirates vs Kuzan. I agree that nobody is getting mid-diffed in this fight, that would probably put Kuzan or BB a tier above the loser....Well let's be honest nobody believes that.
His crew was present at marineford too though, so it goes both ways. And I agree with you, at the very least it will be high to extreme diff since Kuzan=fleet admiral. If BB>>>Ace who's around Jozu level, then Yami Teach was around admiral level. This is not even mentioning how
insane his durability is. And this is pre-TS. Now factor in how powerful of a fruit the Gura Gura is, and factor in 2 years of experience with it and he should easily jump a
huge amount power-wise. He's practically a user with the;
1. Highest potential for destructive ability with Gura Gura and island level Yami Yami no Mi.
2. Around the highest durability.
3. Negates all DF powers, so anyone reliant on DF becomes useless.
His combination is overpowered. I genuinely don't see how it is possible for anyone to ever beat him if he reaches full potential without ass-pull power-ups. The only way to fight his Yami effectively is to stay at a range. But good luck fighting at range with his Gura Gura. And even in the situation you come close, he tanks the hit and takes away your DF. Talk about OP.