Nice! I love tinkering with Ion's.
I would actually be interested to see a real life efficiency comparison between the orange nano and the dual-o-ringed bolt, I would expect the nano to be able to run at slightly lower pressure for the same velocity, but in reality it may not make a huge difference.
As for pods per 68/4500, I can't really speak to that, but the efficiency shouldn't be much less than an electro powered ion.
If you want the best possible efficiency, grab one of Yoda's Closed-Bolt Conversion kits, at 270 psi, those can theoretically shoot over 2200 balls per 68/45.
I recently put together a mech closed-bolt ion, and though I haven't been able to take it to the field yet, I love it, very different sound signature, but shoots nicely