The ecu will change your fuel maps, timing etc, get the most out of what you have.. You will get more gain out of it than a fmic alone. What might be a better option for the time being is an R34 gtt standard sidemount cooler, I'm pretty sure they are still bigger than the S2 R33s, and can be picked up fairly cheap, and you cant see them. So less chance of the police picking you up for it, and people nicking your car.
Id still go the ecu tho.