But he seems to think the stock header is pretty good and has a little better midrange.
It does. The C14 was designed for more lower range power than the ZX14 and the ZX14 more high-end power.
I want the cat gone (and its heat) and I think it will flow better and make better sound as well.
Sound- don't know. But previous research postings showed essentially no change in performance with the cat removed. Heat- some, probably. The C14 doesn't have much of any issue with flow in the stock exhaust system (unless you are trying to give up lower-end power for higher-end power).
One thing you don't see on a dyno is the impressive throttle response after a reflash (since that is more of a time function). But that is seen on the 1/4 mile.