you can get to around 300 - 350 with a remap, exhaust, 100 cell sportcat or decat, induction kit, turbo manifold, hybrid turbo, forge actuator ran direct and uprated recirc valve. (also need an ap organic clutch or similair for the power)

all in prob around £2000 - £2500, could cost alot less if you use 2nd hand parts, could cost alot more if you go for expensive exhaust parts like primary design.

to get 350 - 400 you need uprated injectors, fuel pressure regulator, fuel pump then start looking at internals, air to air, e.t.c if you want more then that