I use to have a YFZ450 and those things are fast as hell stock. The only thing I didn't like about it, is how it handled. It felt like you were riding a 2x4. I sold it and bought a Honda 400ex which in my opinion are better.
My uncle has a Predator and I didn't care much for those. They're fast but they're "too" big for my taste. They remind me too much of the Yamaha Raptors, which is why I hate Raptors because they're wide and easy to tip around turns, and they can't climb hills worth a shit. 400ex is a good racing bike and back road bike.
I'd recommend a 400ex if you've ever rode one before. I just rode a 450R last month for the first time and they're a beast. Bad thing about those, if you want them to be fast you have to do a lot of work & money into it, which is what my friend did. I wouldn't want one unless I had loads of money to put into it.
But yea, why not a 400ex?