Sure, the PC pipe is much better than stock. The PC pipe also seems to be a good choice for MX bikes and KDX 250's. Also, it's no secret that Pro Circuit makes awesome products.
My experience with KDX's includes '95 to present KDX 200's and 220's and that experience is quite extensive. On these bikes (mine and friends') I have used the PC, FMF T pipe and FMF R pipe. Compared to either FMF pipe, the PC is a waste of cash, especially on a 220. When installed on a 220, the PC pipe closely mimmicks the powerband of a KDX 200 but with a slightly stronger midrange. Now, why own a 220 if you want to give up that nice, off idle snap? On a KDX 200, the PC pipe takes an already weak low end and makes it weaker. Sure, there is a nice power increase everywhere else but ride the tight here in NH with that sucker and your clutch finger will get tired real quick. The PC pipe might be a blast in the desert but who on earth would want to wind up a KDX to a screaming speed out in the open for any length of time? Plus, the FMF Rev will give even more top end for you desert speed junkies.
IMHO, there is absolutely no good reason to install a PC pipe on a '95 or newer KDX and if you have a chance to try out the other pipes you'll see why.
Before you get all riled at me for knocking your new purchase, just remember: you did ask :D