Don't over estimate the ramming effect. It is negligible, unless you fly real fast. You may see a 1% power increase, at the cost of risking your engine going lean.
See the calculator here! http://www.wallaceracing.com/ram-air-calc.php

PS
A 1% power increase at 100 Mph will not be noticed by a normal person. If the scoop is located in a low pressure zone which the cowl very often is, there will be no gain at all, since the pressure lowering will cancel all ram air effects.
In pylon racing, a slight cornering mistake will nullify the gain pretty quickly.

PS PS
In a two stroke, a denser crankcase filling does not automatically translate in more power, unless the muffler system can harness the extra gas flow and prevent it from going out as fast as it came into the cylinder. Four strokes do not suffer from this.