I tried adding more oil for a day of riding just before sending my forks out to Pro-Action that following Monday. It helped a little bit, but there wasn't any huge difference. I only added 5ml in each fork (stock springs '99 200), everything that I've read states that if you add too much oil with soft springs the produced effect is an initial harsh feeling followed by a sudden blow through the remaining travel. A Tech Tips section from a Dirt Rider magazine (probably a 4 or 5 year old issue) recomended adding fork oil in 5ml increments to avoid the described effect. A friend of mine experienced this very problem after he added too much oil to the forks of his '94 WR250. While trail riding, the bike gave you arm pump like an old KX500, and blew through travel like an 80 hitting a flat landing. I imagine that these effects would be more apparent on a KDX since they dont have rebound clickers. I recomend that you add 5ml to each tube, go in a few clicks on the compression clickers, and bleed the forks before each ride (see your owner's manual). It should be OK for trails, but I weigh in at 190 as well, and I assure you that fork mods will make an incredible amount of difference. Look into installing the Race Tech Gold Valves or heavier springs yourself, you'll save about $150 ($190 with the springs) over the price of a race shop's mods, and your investment would produce better overall results than any engine or exhaust mods.
-Nick