I did just about exactaly what you're planning on doing. I used to have a 97 kdx 220 with a FMF pipe and silencer. I moved from that to a 2001 yz250f. The 250f is better in EVERY way. It has WAY more power, you just have to open that throttle up. If you try to lug your way through everything then the kdx will seem like it has more power. If you ride a yz250f with the throttle opened and the rpms high then it will absolutely kill a kdx. To ride my yz250f effectively in the woods, I've found that I just have to simply be in a lower gear than usual. In sections where I would be using 2nd and 3rd on the kdx, I'm using 1st and 2nd on the 250f and letting the bike rev. The suspension on the Yamahas is also far better than the kdx's suspension. The wrf will have even better suspension for woods than my yzf does as well. The egronomics on the Yamahas are more modern and more suited for trails where you can carry some speed. They are far more rigid and stable. They will corner in bermed and rutted corners better as well. The kdx will have the advantage snaking through really tight stuff. The Yamahas will have the advantage everywhere else, especially on an MX track. If I were you, I would drop that kdx as fast as you can and pick up either a Yamaha wr250f or a Honda crf250x. Which one you get is an entire thread on it's own. Personally, I love Yamaha's but if I was buying new it would be a Honda. I would also stay away from 450s for a while yet. I've ridden a few and they are too much for woods in my opinion. The power on them is great, really nice and smooth but it is hard to run them fast in the tight stuff because of the weight, high center of gravity and gyro effect of the huge engine. Until you can run a 250f wide open through the trails you ride on, you don't need a bigger bike IMO. Plus, look at the top GNCC riders and top riders in most of the other series, they almost all ride 250 2 stroke bikes. They have nearly the power of a 450 with the light weight of a 250 four stroke. They also snake through the trees better than a 250f or 450f too because of the small engine mass IMO. Hope that helps.