I don't think you could compete on any serious level on a KDX in motocross but I see plenty of them on the gate in the 250 B & C classes at my local track. There's 1 guy who races 1 in the 250 A class.. So, YES, you can race one, you just have to be that much better a rider to compete against a full-blown MX machine. I ride mine at the track a good bit.. I've never raced MX, I race the GNCC series. Riding the track helps me work on jumping and cornering plus.. I'm getting better and big air kicks A$$, unless you crash. I would, however, recommend doing the basic mods (pipe,silencer,reeds,jetting,airboxlid,set suspension) before you get serious about MX. Don't let anybody tell you you can't ride it on a MX track, go ahead.
As for how much air before you bottom the suspension or cause damage.. That depends on how you land.. let me give you an example.. Yesterday my buddy blew out the oil seals in the forks on his YZ125 at our local track, he was coming up a little short on some of the tabletops and landing flat, he wasn't smooth over the jumps, he would land with his front tire off the ground and it would slam down alot... I however, did not blow my seals and was jumping farther, I'd say the largest jump I did was maybe a 45 ft. step-down and a 40 ft. table-top, I was totally clearing it, landing smoothly.. SO, it depende on how you land.. Trying to clear a 60 ft tabletop and coming up short at 45 feet every time..BAD Clearing a 45 ft. tabletop and landing smoothly, NO PROB.