To be totally honest it's going to be down to the rider really IMHO, between me, the monkey and a few of our friends we've owned just about all the sporty grey imports MC18,21,28, NC29,30,35, RGVs, GSXR400s etc. and they're all pretty much as fast as each other and handle better than most can exploit.

But if your both really fast I'd have thought the '21 ridden really well would be ahead of a CBR400 with just a can, but the CBR will be easier to ride fairly fast if your a bit more reserved.

When I sold my Aprilia and got a CBR400 it seemed nearly as fast riding at 70% as it did riding at 100% where as on the 2/ riding at 70% was about as quick as walking.

But this is irrelevant as your bike is probably the best sportsbike ever made (and that coming from a '28se owner!) and his is a small FireBlade.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread