I have built several KDX's over the past 15 years. You should definitely do the pipe, silencer, rejet and lose the airbox lid. I have had my suspension revalved and resprung, and it really helped. I have had the suspension work done by Fredette and by Factory Connection, the results are about the same-a great improvement.
There is no rebound dampening on the fork, only compression dampening.
I am 6'5", 235. I went with tall seat foam, CR Hi Bend bars, and spun the handlebar mounts 180 degrees for a roomier feel.
I have recently switched to a KTM 450, but I logged thousands of miles of Trail Rides, Turkey Runs and a dozen enduros over the past 10 years. The KDX has the highest Fun Factor of anything I have owned to date.
Good Luck- And have some fun!!!