On a side note, the cyclone and other impeller systems don't use that much air at all. A gravity hopper that had no resistance to air flow to cause a popcorn would use a lot of excess air. You would have to find a low flow low psi reg to make it work, and it would end up costinf more than an electro hopper.
Here is the thread on the tippmann forum I frequent.. As you can see the cyclone only deducts around 1 fps of velocity versus a sealed reciever. It does use a little gas, but it's not the number that people make it out to be.
On the suction hopper note. I think you would have better luck with a hopper that with a air bladder in the top, that filled up as balls emptied it. That would keep the open space avial. unusable, so then the balls did popcorn, they wouldnt have anywhere to go. Or even just a positively charged air pressure into the top of the hopper.
No matter what you do though, a a gravity powered or even adgitated hopper has it's limits, becauce the acceleration of gravity is the same for all objects. The only way to get a real high rof (over 13 or so) is to force feed balls ie. Rotor or cyclone.
I know the cyclone can only shoot 15 bps stock, but modded out people have reported upper 20s