Burtondr---have you tried going up to 15wt and adding about an inch longer spacer than stock? I did this, it helped some, no leaks yet. You should be able to bottom out pretty hard repeatedly with out self-destructing the forks.
I'd like track down some info on either swapping tt350 tubes and rebuild or better yet, is it possible to do tripple trees and forks off of a tt500/xt500 or ???? If so the same info for the back would be in order (most likely have to swap swing arms) AND you would most likely want to beef up the frame. All in all it is what it is, if you're out riding the XT perhaps it's time to upgrade the whole bike.....
Skidmarx---thanks for the reply. I figured as much. I only have 1600 miles on the bike but they've been rough, ass-hauling miles. Not too easy on the ole XT. I parked it for a number of months and began riding a yamaha powered street bike thumper. When I came back to the XT it seemed like the engine was kinda "pingy" sounding in the top end so that's why I was wondering about valve clearances. Perhaps it's always been like this and I was just forgot. Does this pingy sound familiar to anyone? Any more info on checking the valves on an XT350?