Computer is tuned differently and also has some small wiring changes to the pin out on the
ECU.
IIRC you need to swap the wires for the cam and crank angle sensors on the
ECU plugs. I think there maybe something else too, but I cant remember.
do a search for Power FC into a 99/00 WRX, as the 99/00 Power FC is wired the same as a STI
ECU and you need to make some pin changes to make it work in a normal WRX.