Re: 2 questions for a new guy.
If you were to use a MNH style glass screen then I would assume you are probably not getting the optimal benefit if you are using a Phunnel, Vortex, or Crown Jewel since you are pulling most of your air down the center and not over/through the baking shisha. I don't know how much difference it would make but I believe it would be a "Non-Zero" percentage.
However if you are using an Egyptian, mod bowl, Phunnel with Scalli, or Crown Classic then the screen shouldn't effect you at all since you are still drawing the air over/through the shisha.
Oh... and for more info on the Venturi Effect: http://en.wikipedia.org/wiki/Venturi_effect
34" Km Americana Trimetal, KM Jumbo Base, Crown Jewel Bowl, Blue v3 Narbish, CH "Blue Lollipop" Tip
24" Solid Al Fakher, AF hose, MNH Phunnel
30" Solid Al Fakher, AF Hose, Vortex, Blue v3 Narbish, "The Circus" Crown Hookahs Tip
36" Solid Al Fakher, Blue Razan
29" KM Trimetal, KM "Red Lion" Gilded Base, Crown Jewel Bowl, Red v3 Narbish, CH "Solar Flare" Tip