Might be a daft question but would a easy way of increasing WRX power just be fit a STI ECU from the same age car?

 

No

 

If its a new age then downpipe, up pipe, internals, heads, turbo etc would be different.

A better way would be to change exhaust, panel filter and then map. If you want to go further up pipe and headers could also be changed. But don't forget remap

 

ok did not think it would just thought it might just be like having a PPP map