Stg1 is just filter, exhuast, stg1 chip, 15psi (or more if live mapped) boost, will give between 260 and 300bhp depending on chip (or more if its live mapped).

Stg2- Ignore it, waste of time.

Stg3- Stg1 plus RS500 intercooler, green inj, more boost (23-25psi), suitable chip or live map, ideally a bigger turbo, and maybe a uprated HG if can be bothered. Gives 300-330bhp.

Beyond that its up to you.

Standard crank and rods are good for 600bhp+

Std pistons are 500bhp+ ones really, but best off dropping the compression so can run even more boost on pump fuel.

Non ported head is still good for 400odd, as is std cams really IIRC.

2wd exhaust manifold is fine even at 500bhp+

Std inlet is good for 400bhp+ too. Std throttle body good for well over 500bhp.

Std fuel pump is good for night on 400bhp IIRC but will prob be getting tied now.

Managment on cossies is live mappable as standard, so no issues there, can easily upgrade to 4x4 Cos L8 managment or Esc Cos P8 managment with next to no mods, and then you got options to run anti lag, water inj, coilpack ign, etc etc without spending silly money on other ECUs.

Cossies are remarkably simple to tune, and decent prices, and even some mappers who can map properly too, which is rare, lol.