Originally Posted by StockTalon
A phenolic one will help keep heat out of the intake manifold since, most cars have coolant lines running through the throttle body. It also, indirectly makes the plenum larger.
The time spent making it isn't worth the gains you'd get from it. Lengthening the intake runners with a phenolic spacer, you could fab this using a gasket to match the ports, would likely be a better option. Until someone tries it, we'll never know.