Well, I do not actually have my "new" KDX yet, but speaking from experience from my '96, yes, there was definately a "hit" when the valve opened. The pipe and RAD valve surely contributed to it. Anyway, I can remember becoming just a bit irritated that the valve would stay closed just a little bit too long, as it started revving into the top end. I hope it is not just me, but I could actually hear the change in pitch when the valve opened. At one point, it bothered me enough that I pulled the whole top end apart to clean everything. That helped, but I could still feel the valve lagging behind just a bit.
Another thing that I noticed was that, as the valve parts got dirty, the valve would open very late until all of the oily gunk that was coating the parts was warmed up to temperature (usually a good 5 or 10 minutes of riding).
Just before I sold the bike, I was considering pulling the top end apart again, cleaning everything really well, then spraying them with Pam or some other anti-stick cooking formula. I never got the chance to try it, though. Perhaps with my new KDX, I will do this right off the bat.