With the mods the safe flow for your setup would only be around 220bhp max.
Limiting factor would be the down pipe and cat, they get stupid hot with just stage1 power, which means the engine had to run rich at the top end to stop thermal damage.
To get good power you will been more than a cat back milltek, 007p and air filter, with just these mods running 270bhp your going to melt parts, you will need a fmic and performance down pipe and sports cat / decat.
Stage1 aum ko3s power safe is 210bhp/230lbft 19psi peak and stage2 with supporting mods would be upto 240bhp/270lbft 23psi peak
Which ko4 are you looking at using?
001 or 02x?
What engine code are you looking at tuning? Aum with stock injectors and maf will flow 270bhp, but to make the most of the k04 hotside which can produce awesome torque it's a good idea to use the TT 380cc injectors. If it's an AGU then the injectors and maf top out 240bhp As the map mag scale is only 673kg/hr so a vr6 maf and 310cc injectors would be needed.
Your best option is get the most from your current turbo, ie stage2 power with turbo back and fmic, then go ko4, but going ko4 with out down pipe is going to be a waste of money.
