Seems like it. Zaraki is already above Captain level and V has yet to use Complete Holy Form (which greatly enhances their ability)

http://readms.com/r/bleach/573/2297/1

It does make sense, imagination has no limits. Or rather it's only limited by the individual's of imagination.

I don't think he is the strongest. I think thats more of a self proclamation.

And imagination has limits. All Kempachi has to do is make him doubt himself.

Also, his imagine power appears to be based on the opponent beleving it as well. If you have somene like Kempachi who 100% belives in his own power, a lot of the things he imagines (like cookie bones) won't actualy work.

I predict this battle is going to go in a way similar to this....

Steinritter does some attack that should shock or even frighten most people, for example Kenpachi losing an arm. Meanwhile Kenpachi will completly disregard it as dead weight and be unphased. This type of reaction actually puts the stinritter on edge asking questions such as "What are you?" as opposed to who are you, at which point Kenpachi willrespond by cutting through some imaginative illusion and the stienritter while replying his name.