Gravity flow might work, but I wouldn't go without the pump. Here's why:
I know you need 4 ft of height just to transfer through a Therminator via gravity. Add additional height to account for the oxygen stone and the fill height of your fermentor and you're looking at a pretty cumbersome set up.
Just invest in the pump or use the one you already have. That way, you won't ever have an issue with flow, especially as your system changes.
As far as the pore size, MoreBeer claims you can get 12-15 ppm with the pro setup (the tri-clamp model - is this what you're referring to?). I trust MoreBeer, but you can always buy it, set it up, run a high gravity batch through it, and test ppm with a (graciously borrowed) DO meter.
It makes sense - you may have bigger bubbles, but you have a higher pressure (from the pump) and more residence time (through piping into tank) to force those bubbles into solution.