Any gains are going to be heresay. The stock throttle body will flow good enough to support more than twice what the motor is putting out in stock form. You will never notice the difference unless you compliment that mod with some headwork, fuel delivery, and exhaust. You probably still wont notice much from the vg30 unless you have a turbo or s/c vg30. Take my reply and use it, ive done several 60mm throttle body swaps on the older z's with L28 motors. Its always the same old stuff, grinding the manifold, and my friends always complaining that theres no difference. The only thing thats gonna happen is that air velocity is going to be reduced installing a bigger throttle body on an engine that doesnt need it. I think Nissan did a good job matching engine components to flow evenly.
in short (cliff's notes), dont bother.