It's supposed to keep the upper intake a little cooler thus making the incoming air have a higher air charge due to the cooler temps... I don't know how much it'd actually add... I figure the overall heat of the engine would still heat the manifold anyway?
Personally I would save the money and have tmoss port the lower since that is the big restriction on the car. I don't think that just puttin a spacer on the stock intake is really gonna give you any noticable performance gains. Even with the spacer you are still running hot exhaust gases through the upper in the internal EGR passage. So the upper will still get pretty warm. Until I blocked of my EGR passages at the cylinder heads my GT40 upper would get pretty warm and I am running a half inch spacer.
If you wanna cool down your upper intake, just get a bag of ice and throw it on the upper. spacers dont insulate crap, they just make a very, very marginal decrease in the heat transfer rate from the lower to the upper, so what I mean is that it takes a little longer for the upper manifold to reach the same operating temp as the rest of the engine. I have yet to see any dyno testing of one to verify what they are supposed to do, or better yet anybody actually measure the temperature of the manifold before and after the spacer to see if the upper actually runs cooler. I'd bet $50 its all