guys running 318bhp, 303lbs in a 99 uk turbo fitted with

04 uk wrx lump
550 injectors
vf28
uprated headers
hybrid fmic
k & n
fpr
walbro fuel pump
full decat system
apexi pfc + avc-r

was thinking of going for a larger turbo looking at a 20g aiming for 380bhp or there abouts as the engine should be ok for the power output.
the gearbox may or may not hold up but i have been told that the 20g would see around 350-360lb so depending on how it is treated it should hold out.

i assume i willl need to uprate the clutch and might as well do the flywheel at the same time but as and when (or if lucky) if the gearbox does and then i will fit a uk 6 speed, can i still use the clutch and flywheel, will they fit as i dont want to have to buy new 6 spedd clutch etc, as if they dont match i will hold out untill i got the cash to do it all at once

thanks

steve