As i have modded the previous R33 and now that i have an R34 with some mod plans to do, ill share with you...
1) Full Turbo Back Exhaust => Preferably 3" dump pipe, 3-4" hi flow cat, 3-3.5" cat back exhaust with high flow muffler. (+15-20kwrw)
2) Air filter => Apexi has best filtration, K&N is pretty good too but not as good as apexi but has a louder woosh sound if u like. +5-10kw
I believe there is real no need for an FMIC upgrade as the intercooler for GTT is efficient enuff to make up to 200kwrw unless if ur aiming above that. So i skip to...
3) ECU Upgrade => Cheapest is the piggyback styles such as Apexi Super Air Flow Convertor or controller...or if u have the budget it'd be best to go for an Apexi Power FC if still available somewhere as they are discontinued... (+20kwrw)
These are prob one of the most basic mods you can do to have you running around 200kwrw on a GTT R34, but you might have to change ur clutch and fuel pumps and etc as well...