My 2003 Blob has full decat exhaust (Not including up-pipe, that i still need to do) highflow panel filter (Pipercross) and has been re-mapped to 266.1 BHP
Thats on standard WRX Turbo TD04 , which is more or less topped out at that, you could go a bit closer to 300 but thats a good number.
if you wanna go further then get pink injectors a walbro 255 fuel pump and the bigger turbo from the STI, something like a VF34 or VF35 :)
Hope this Helps
BTW the Ninja Backbox on sounds awesome FYI