To be honest with you -

A good running mildly tuned Zilla is plenty fast. Spend your money on making it reliable first. You start throwing money at big pipes, you start to find big problems like Dez said.

I got my first Zilla as a non runner. Put a big pipe on it, porting, etc; and its a beast. Not really a trail bike anymore. I picked up another Zilla that's stock. Its pretty milked out but is still a strong runner. I have a YFZ450 as a comparison.