I know I've sort of hijacked this thread, but I thought I would post a quick update. I've installed the XP3 with the outflow going up to the in-line heater, then over to the top of my homebrew CO2 reactor, down through the reactor and out the bottom, then up to the tank. The XP3 came with _just_ barely enough hose to make everything work (more on order for mods/additional plumbing).
The priming went exactly according to the manual. The only issue I had when I started the filter was a lot of "running water" sound from the reactor. Obviously, there was a good bit of air trapped in there. I planned on detatching it from the bracket today to move it around so the air could escape, but by this morning there was no more noise. All of the air has been flushed.
I shut down and removed the filter (using the quick disconnects) to add some media from my other tank, and upon reconnection it primed and started just fine. The only problem I have now is that my in-line heater isn't perfectly vertical (about 30 degrees off), so I have to be sure to clear it of air bubbles before I turn it on. Once I get the additional hose, I will move it to the vertical run after the reactor. Yet another case of "act first, think second" syndrome.
So far things seem to be working quite well. I'm also very glad that I used a check valve as the CO2 inlet to the reactor.