is it possible to / would there be any gains if the throttle body was replaced by one out of a 300zx?

 

The 300ZX uses two throttlebodies that are about the size as the Maxima TB, if I'm not mistaken. So... no, it won't do you any good.

 

You can replace it with a pathfinder TB. Make sure you read up on it first though, I think it involves boring where the TB attaches and possibly something tricky with sensors. I'm also not sure whether or not it gives any gains if you're not supercharged.