Ive been doing it for 4 years now on my M4. Non ported cylinder for a 350+mm barrel, just under 400 FPS. Swap in a shorter barrel for indoors, and it drops to 335fps. Accuracy is excellent on both set ups, I can even run the shorter barrel outside, you loose range clearly, but accuracy is just fine.

I hear it wears out parts more, since your putting more air through then you need, but I havnt seen it in all 4 years, same major parts.

that thought had crossed my mind.... but I need a whole p90 body and gearbox to do so :P