Aaron, I’d recommend getting hold of a GTSt ECU and plugging it into your car to see if you get any error codes…I’m 99% sure after looking at the wiring diagrams that the ECU won’t see ANY difference…unlike the R34 ECU *sigh*
Just get a loaner, it’ll only take a few minutes to swap over and see, ideally just swapping the ECU and dropping the base timing a few degrees may be all you need to do.