You're gonna LOVE that YZ!! NICE bike! I rode a buddy's last fall, and it's got the stuff to be good. After you get it dialed, you're going to look back and wonder WHAT in the heck were you ever doing on that KDX??!
I had a very well set up KDX220, and I went instantly faster aboard a 01 RM250.
One thing you'll note, is that the 250 motobikes aren't for just everybody, but it sounds like you've got the riding skills to apply the extra abilities of the YZ.
Good luck racing!!
BTW, Tom.... You must have never ridden a current 250MX bike. The new crop of 250's have more low end power than a KDX could dream of. Top end, there is absolutely no comparison, let alone compare to the 250's handling in the fast pace snarley stuff. How can plenty of power be "horrible", unless you're going REALLY slow?