I'm trying to find out if a Plenum Spacer would be a benefit for my car. I want better performance and fuel economy. I've heard of lost low end torque with some applications. Some companies are making HP gains claims of 8 HP with and 2 MPG increase with a stock setup. Also I've seem different thickness of material and some kits have extra gaskets and a shut of valve. What is the real world expectations from these kits? Why one over the other?

Plenum spacer? What's that? I've owned my G for awhile now and I've never heard of such a thing. It's probably some snake oil product.

A spacer works great for power gains on the standard VQ35DE motors but you have a revup motor which barely benefits from a spacer. Your butt dyno will tell you it works but a real dyno will really only show a change in timing. The MREV2, however, will give you an extra 15lbs of torque but costs a bit more.

There's BLOX Racing 5/16,,,Motordyne 5/16... Skunk 2 5/16,,, Are All 5/16 Plenum Spacer of the same quality? How effective is that thermal gasket and shut off valve to the throttle body? 1HP gain maybe? Pros& Cons Please!

OP, unless you purchase a non-revup lower plenum from 03-04 motors (MREV) or MREV2 then the spacer will not benefit your stock setup.
The MREV2 is kinda pricey around 400 so if you want a cheaper route, find an 03-04 motor and take the lower plenum from that, you will notice the differences in the runners.

Op I did the mrev2 and the 5/16 plenum spacer and love it... Is worth the money..

Oh nm its in his title didn't see that