2mm of gap is too much space to have inbetween your blades and shroud. You will definitely lose alot of velocity. on the flip side being to close is no good either. I strive for about 1-1.2mm distance. Now, take notice that this is in a "perfect" world. Given the fact that our ducting is not aerospace engineered for the most part. Now the cross sectional area of the intake needs to be the same as the exhaust. Unfortunately this doesnt work all the time. Most of out intake ducts are divergent. While the exhaust is usually a convergent meaning it shrinks. This does increases flow across the fan and reduces pressure at the inlet. It still causes a loss of flow at the blades, so try to strive for a smaller area of gap between the fan and shroud to maximize velocity and minimize air loss.