I had a KDX220 and my YZF concurrently, and I finally sold the KDX after it sat in the garage for 2 months. I knew the KDX was doomed after I rode the two bikes back-to-back at Dirtweek.
I want a WR250F. I'm 5'11", 195 lbs., and my YZ pulls me around very well. I need the WR's gearing for woods work though. The YZ feels like it has a 60mph 1st gear, and it's hard to ride in the tight stuff. It also suffers on slow, technical hillclimbs. I don't think the problems can be solved with sprocket changes.
If you buy a WR250F, I'll be happy to ride it..uh, evaluate it for you.