The catted uppipes were only in the bugeye wrx and earlier blob wrx (pre-2005 rings a bell). STI's always had a decat uppipe. With your current turbo you can get up to about 280-285bhp with decat uppipe, decent panel filter, decat downpipe/centre pipe and a map.
To go further than that the easiest route is VF35 and pink injectors from newage STI. Once mapped (assuming full decat etc) you would see ~330bhp.
Worth changing the fuel pump to a walbro 255 whatever way you go, if your car is still on the original.
