The slight FPS loss is usually due to chronoing the gun an extra 8" away from the chrono
The pressure difference & turbulent air should be really negligible on the fps
Anyway, a new concept, most people have no idea what LENGTH of suppressor is actually required!
For actual silencing purposes, it seems you don't actually need a long suppressor.
My VSR only uses about 35mm of it's suppressor (G-spec with a 430mm barrel) and it's dead silent. I figure that's around the minimum length.
All you really need to do is catch the deflected air blast coming out of the barrel, and it comes out at a pretty steep angle, so you don't need a foot long silencer.
It's not like real steel where you're trying to catch a significant portion of high pressure gas and slow it down, you're catching a very small amount of very slightly pressurized gas.