Its hard to reach any conclusions regarding the testing of VE enhacing parts without knowing that the set up you are working with is optimzed and has no existing weak links or bottle necks. End users must remember always that there are only 5 ways to make power
1 - Displacement
2 - RPM
3 - Volumetric Efficency - (filling the clyinder more completely with a charge)
4 - Density (cooling and increasing pressure)
5 - Chemcial - e.g. nos, etc.
Clearly all these kind of modifications like intake manifolds, TGV deletes, exhuast , etc etc etc are only effective in enhacing VE by allowing the intake charge a easier flow into the engine and by allowing the exhuast gases less back pressure to evacuate the cylinder. In most cases we are working well below 100% VE on a Subaru engine which yields an effective displacement well below the 2.0 or 2.5 L we have to work with.
Any VE enhancement will only achive its full effect if the rest of the intake and exhuast system is fully optimized. If the rest of your system is not breathing perefctly, then you won't realize the full VE gain of an intake manifold.
In simple laymans terms its like having a series of bottle necks or restrictions and eliminating one of the kinks in your flow. It still has the other kinks left holding you back.
Last week, I installed a Cosworth Intake on a 06 STI with a Dominator 2.5. After a short period of testing and trying to make additional power, the turbo over spun and blew out a seal. Although we did not realize ANY gains from the Cosworth intake on that turbo, it was clear that the turbo was the limiting factor and could not flow much beyond 400 whp no matter what intake manifold was on the car.
All I can say with 100% certainty is that the Cosworth Intake flows significantly more air than the stock one and it will pick up power on any car which is exceeding the air flow capacity of the stock intake manifold. I made over 80 whp.
Clearly, a intake maniold is not going to do much on say a Stage II car as the air flow is not impeeded by the stock manifold. As you work towards a more optimal VE configuration with better head design, cams, exhuast, fmic, etc etc the intake manfold upgrade will start to yield more and more of a gain or increase. Only on a car with everything else optimzed will you see maximum gains on the change of an intake manifold.