Hey Trev. Im really not sure about this to be honest I just remember someone talking about it before. The connectors may be different and the sti has stronger internals and a higher rev limit and I think different cam profilies. You would really wanna talk to someone who has done it before and ran the engine for some time without damage. Me myself wouldent go for it at all id much prefer a new ecu and map and know its safe. I cant see an ECU from a different spec car being safe at all.
Is it for extra perforamnce you where thinking of this.
Z4 ECU is 1994-1996 WRX and the Z5 was suppose to be alike.