Hey, heres not a lot to get from the xt without diving into the engine.
The engine is made bulletproff, so overything is bigger and heavier.
You can make the bike more rideable, but you can also do that by openening youre stock exhasut, open the airbox and install a new airfilter, and jet accordingly.
If you go with a new header and an exhaust, do it for the looks, and see it as a + if you get a little more umf.
Oversize stainless headers
Laser produro exhaust (opened up fully by the previous owner)
Opened up airbox
Pipercross foam airfilter.
If you do that or simular, heres a good estimate of the jetting:
+1 size in idle jet
Raise both needles 1 notch (lower the clip)
+1 on both primary jets.
Adjust Co screw
Havent tried the dynojetkits, so cant answer fot thoose.
Sometimes, i think, that for the money invested in pipes and shit, I've could have bought a 14T cs sprocket, and a new shock, and ride faster.
Consider what you want, and let us know.