As much as i like the idea of the Blowback idea i also would love a turny were Mags and Cockers were king again. The Revi idea i love but sadly revies have not been offered for many many years and not every one could get one. I think if you put a limit on hoppers at all it have to be a shake and shoot only rule other wise you are limiting too much on equipment.
Guns like the Tippmann A5 with a built in hopper should not be allowed simply because it is a force feed hopper. I dont think (i have owned two A5s) that the Cyclone feed is a large advantage.
I can personally gurrentee if a Mech only turny popped up around here i would join in. I may shoot my RT ULE Custom Automag or my Classic 68 Automag or maybe even my Tippmann Pro/Carbine.
Another thing id question is if you are allowing mech only guns would you allow Pumps. Obviously pumps are at a dissadvantage even compared to some blowbacksand there are pump only games and sutch but what damage can allowing them in mech only games hurt.