I've got a vertex XL system with a single quad diffusor that I installed at 24' in my approx. 1 acre lake early this year, and it is doing a beautifull job. At start up it was running just below 5 PSI. I checked the PSI this morning and it was running at approximately 7 - 8 psi. Is this an indication of anything I need to keep an eye on?

If the water level didn't change, I'd expect some growth on the diffuser. If you haven't done so, try shutting it off for a short enough time to stop air from going thru the diffuser, and turn it back on. That might clean out some of the slits.