Since I purchased my stock gtt 2 years ago, this may give you an idea.
1. Installed FMIC, 3" cat back . dyno result 175kw
2. Installed Apexi PFC and Apexi BC, Apexi Pod filter . tuned dyno result 189kw - ran out of fuel pressure and injectors maxed out anyway at 12psi
3. Installed Walbro fuel pump, coil packs, high flow injectors, Z32 AFM, front & dump pipe @15psi, tuned dyno result 212kw
Now in the market for a turbo upgrade before the stock turbo gives up otherwise very happy with this result, car is very nice to drive.