I have ridden, stock, pipe and stock arrestor, and gnarly and TCII. The Gnarly is a must because of the strength. I would also get a pipe guard from Holeshot. I looped my 300 on a step up in the middle of a hillclimb and the pipe (bike) landed on a large rock, virtually no damage to speak of. The TCII has a very mellow, but cool sound, saves about 3 or 4 lbs minimum, looks worlds better, and doesn't spooge as bad as stock.
But as far as 'mods' go the best investment for performance is the Delta II V-force reed cage.