As said the classic impreza needs a daughter board to remap, do you know what turbo its on?

mine a 2001 bugeye at 240hp, and if i do a full decat and remap on the TD04 ill see about 270-280bhp and 320lbft, but my ecu can be remapped as it is.

i take it he went to engine tuner (alan jeffery) in plymouth? they are pretty good with imprezas, so if they are saying its 290hp then it must be near standard!

for the claimed power he was running he will need a different turbo, but there are so many that fit you would need to know exactly what he has to determine wether or not the car could make the power suggested