Renji trained Chad pre-Hueco Mundo if I'm not mistaken. At that point in the story I think Chad's power was still behind Renji, but not by much. However, if he managed to destroy Zabaimaru (Renji hadn't mastered his Bankai yet), he'd win against Renji extreme diff
Chad here fights Renji the first time he used his bankai against Byakuya so he didn't master it very well.
I think Chad became a little stronger than this Renji. It will be a very difficult fight but I think he can tank with his shield a baboon beam. And Chad can spam good beams with his shield arm and his other arm can maybe hurt and disassemble Renji bankai.
I think Chad can win very high diff.
He'd win high diff against pure Shikai Ichi (
I agree it is very hard to rank Ichigo because the mange is clearly weird in its power level.
Ichigo post training can destroyed vice captain with his bare hand and no efforts.
But if we consider that a more experienced Ichigo needed bankai against a privaron espada and Chad defeated one too. So a bankai-less Ichigo with much less experience can likely be defeated by Chad.
I said Chad can win high diff.
He will be beaten by first time bankai ichigo around mid diff.
Chad doesn't make it past Ikkaku. Ikkaku is Captain level with his bankai but always held back because he didn't want anyone to see his true power. Chad can speed blitz, but Ikkaku is just as strong if not stronger than Renji.
Ikkaku is a complicated case because when he first used his bankai we can maybe tell that he held back but he has great difficulties against the arrancar and it was a very high diff fight. And that arrancar seemed weaker than a privaron espada.
So if I just use Ikkaku feats I can say that Chad has a chance.