I'm no expert, but IMHO, you are comparing apples to oranges. However, you could hop-up the KDX to run with the KX, I have seen it done. Pipe, carb and jetting that leans to the MX side of power delivery and a top-end geared porting job had a guy I know running with 250's, not 125's. His bike was smoking the 125's in holeshot practices. This was a 200 with a 36mm carb and Race Tech (local guy) porting.
Too put things back into perspective, this guy ended up selling his 200 and purchasing a CR250. Seems that while he could match the power delivery of MX bikes on the track, the suspension could not be easily modified to keep up on the track.
Bottom line, the KDX is fun on the track, but if you want a MXer, you are better off buying one from the start. I'm attempting to engineer my KDX 200 to work decently on and off the track, but not optimally on either. I too am somewhat inhibited by aging and doughnuts....
Hope this helps.. ;)