That's great to hear, all positive updates. 5 psi @1600rpm is nuts! Holy low RPM boost, Batman! Nice work, that up pipe mod is clearly paying itself off.
You can push these turbos up over 15 psi with enough fuel, but they start to fall off the [turbo]map over like 22psi @2600 rpm I think. Don't quote me on that but the point is that they aren't very efficient up at those levels. These aren't the most efficient in the first place. Someday I'll get a drop-in upgrade and a 90cc pump. Can't imagine doing much more than that myself.
But as for charge air cooling-- the air-to-water setups are compact, and I'm sure you could fit that under the hood. I've seen a few good ones around here and FTE and they perform just as well. Better, in some cases.