Im also tring to hit 300 with a 01 GTS right now I have 220.1whp and 198.7lbs tq which should equate to about 255 to the flywheel. stock clutch will be fried at 200whp if you like to drive hard. Right now Im working with a shop to install the xs kit and if you have a gts first thing you need to know is that 11.5 to 1 comp will be hard to turbo if the car is a daily. If you have a gts and want to turbo here are some must haves.
LSD or other company
2. 3mm HG no thicker or you will burn through it ( this will bring the comp down to 9.7 to 10.0 to 1
3. make sure you get the apexi stand alone with the kit I think they only sell it with it but not sure.
4. you must upgrade your fuel pump the best one to use and is a direct replacement will be out of a supra TT (you can get from toyota for about $250 and is good up to 600hp.
5. spend some money for a good clutch/flywheel
I have an RPS 9lb flywheel and their 6 puck sprung clutch and RPS throughout bearing and love them.
6. Plan on upgrading your exaust to allow the turbo to reach max performance.
Outside of the basic things you will do to a turbo a car this will be a good start. By the way the xs kit will make 251.7 whp on a GTS