I agree with Corey, but also if one has a heavily ported upper intake
the spacer may not do anything, or less than an un-ported intake.
I read a very detailed article by David Vizard and he explained heat
transfer in such a way that proves that anything you can do for
reduced heat transfer will help below 3KRPM's give or take. He is
much more well educated than the sceptics and he is right.
I firmly believe though that well-ported uppers pretty much nullify
the spacer's effectiveness. Maybe not 100% but a fully ported
upper has lots less heat transfer due to smoother internal surfaces.
I had a dyno (Josť still does have it) that showed the Stage 3 giving
a nice torque boost up to 2500, dead even after that and gaining again
after 4500 RPM or so. The low end boost must have been due to the
internal polishing, because the runners were shorter and that is supposed
to move the torque curve upward. This was a Stage 1 vs. a Stage 3 dyno.
Vizard states that at RPM ranges above ~3K, the air is not in the intake long
enough for significant heat transfer, and therefore the intake charge heating
is minimized at higher RPM's.