Ive heard this b4 but I believe it was said that more power was gained from being "slightly" higher from the floor, not in the middle of it, and it was actually for N/A applications, not boost.

 

I couldn't find a quick link its something I learned in school doing formula SAE if you want another opinion on it call kinsler fuel injection or velocity of sound. It is basically accepted fact in the industry though. Anywhere between an inch off the floor and dead center is probably ok, mine are dead center because of the distance between the plenum wall and the velocity stack, any lower and the gap would be small enough to cause turbulance.