I'll probably do a flush with radiator cleaner (you recommended Permatex #18).

Yeah, I was a bit surprised at the vented loop vs. no vented loop performance too, but it was pretty clear. The temp climbed up much more quickly, and at a lower RPM, with the vented loop on --we've done this twice. And twice (yesterday) with the loop off, and the engine runs at a somewhat higher (a couple hundred more) RPM without overheating.

Anyway, raw water flow test tomorrow and, as well, probably a flush of the coolant side.