Despite what some people say you won't see gains from a spacer on a dry manifold. If the fuel is introduced before the intake manifold (carbs, TBI's) then yes you will see some gain.

 

They only thing that I see is that, in effect, it increases the overall runner length about an inch, which in theory, would move the rpm powerband down and give a slight increase in low end torque and throttle response.
I haven't seen any dyno numbers and have never tried one to verify. Most of what I have read is that they don't work on a dry manifold.
On a wet manifold, I can see the swirling effect help atomize the fuel.

Severe, did you notice any gains with the MIT intake pipe on the stock box? I've been thinking about getting one myself.