Two words... Race Tech. I'm pretty much your size at 200 pounds. I've enjoied airing my KX out and have no problems with the bottoming. But, I used to have a blown out shock and instead traded out my rear shock for a race tech rebuilt shock for a 180 pound rider, which is great on small/medium tracks and will swap that out for my previous shock I had rebuilt for a 220 pound rider which is better for 100+ foot jumps. I've also heard that MX Tech is also good.
Btw, the KX off the show room floor is valved and sprung pretty much for a 120 pound rider it feels like, way too soft. That is why I like the Gold Valves from Race tech, they have been the best in the MX sciene.. But ask Paul, he does alot of Dune riding and should know about this area. I just keep to the Track and sand drags.