When I put the Z32 afm on my car, it ran bulk rich... All the way from idle to redline... This is not just full throttle, but part throttle too, so unless you've got a full ecu to take advantage of that, you'll probably find that your economy will go to *****. Obviously you'll be able to tune your full throttle with teh afc, but part throttle won't work.
The actual diameter of the AFM between the RB25 and the Z32 afm's are both 80mm, so you're not going to have any less restriction by moving up to the z32.
matt