The best throttle body spacer is the EFI Nitrous Plate LOL. You should pick up about 134rwhp and 241rwtq for about $900.
But as for the regular spacers, you won't see or feel gains. :-( Not a need for them on this car. If one works, then two would work better right?
I've questioned them before, and bought one for my GTO and it does hold down a stack of papers pretty well. 'nough said!
One cool thing you can ACTUALLY do with them on your car is get them custom engraved on the top to say whatever you want, and that can look pretty cool up there with your throttle body for when you are at a carshow, but it all depends on if you want a name plate.