I have Hillclimbed a 96 kx250 and a 97 500 for a some years now. The 500 was stock length and so was my 250 for the first few. Surprisingly, the 500 times were not always faster than the 250 times, and even when they were, it wasn't by much. A 500 seems alot faster because, for one its louder and pulls harder at times, but also spins alot harder at other times. Out on the trails and ridin for good times, I agree lugging the motor will hook it up and show you the difference quickly. I would disagree that the cr 500's are more explosive. they have no powervalve and have less power stock, but I believe this gives them more traction. the powercurve on a dyno has a big jump in power when a kx powervalve opens. That lends to its traction issues... or rut- making skills depending on how you look at it.
During a race / hillclimb / dragrace, the hook-up has alot to do with body position and bike set-up. When spinning too much, you really have to grow a set and scoot back on the seat, but be ready to get the weight off the wheel when going ( i.e. leaning forward or slipping the clutch). My worst bad habit is, I give the 500 too much respect, and ride the tank when I need to go fast. the ideal traction point is ............. The front wheel barely off the ground. Maybe two to three inches max, when you start using body english to carry the front wheel, while holding it wide open in second, then you are fast. to go faster, its a matter of gearing and suspension (and tire) set-up. I wouldn't think of pulling 1st gear holeshot on the 500. The guy to beat in the 600cc class that I race has a worn out spot on his cr500, where the seat meets the back fender. he is also a master at keeping the front wheel light.
The thing that I noticed about the 500 that surprised me, was that the 500 jumped and landed as well as the 250. I thought it would be like driving a streetbike around the woods, but it is really close to the same in that area. I would definately agree that my 250 turns much better than the 500. the 500 will chew up a whoop section though like no other. Now with the extensions on the 500, with some race mods and race gas, i would venture to say that it is the animal that everyone is talking about. I have had 4 runs up the hill on it this year and i would say 3 of those runs were too fast for a good healthy long life.