i only had that problem with the stock shifter...afterward i bought a steeda tri-ax short shifter...i haven't missed since....could possibly be when you are racing, you're not totally engaging the clutch?
Dittos here on my 02 GT with a Tremec...no problems at all. It's stiff when it's cold, but before I even leave the garage, I row thru the gears once or twice, then I am on my way without any issues (except trying to remember to turn the traction control OFF!!!)
It's the crappy tremec 3650. My car won't go into 3rd if I try to shift above 5k RPM's. If i shift it slowly ... it will go in, but shifting slowly causes the engine to drop below the 5,000 RPM range. A lot of people have this problem. Check out Tremec 3650 issue ... threads. I had this problem with stock shifter, and Steeda Tri-ax. We should count our blessings, cuz this is what the better T 3650 trannys do!