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.
The more I've thought about it, and read your posts, I have designed a 3-tier setup where I'll just use gravity for HLT to MLT, and use the pump for wort through chiller, O2, and into fermenter. The in-line oxygenation setup I ordered is just a series of "T's" with an inlet for O2, an inlet for wort, and outlet for wort. The pro series was sold out (and a bit out of my budget for now). The stone pore size (as reported by MoreBeer tech support) is 2-micron. Also, they say you can't change it out, but we'll see when it gets here on Wednesday.
Fortunately I work in a field where I use D.O. meters quite frequently. I can also measure pH, conductivity, temperature, resistivity, salinity, TDS, etc. I'll definitely be checking the D.O. when I brew the first batch with it.