It is not a Subaru Thermostat; a MotoRad brand. It seals fine and I've tested it with a pan of water on the stove.
I did the remove the vent plug deal and filled it ever so slowly. I then let it sit for a while and then topped it off, put back the plug (a royal PITA BTW), put the cap back on, squeezed the upper radiator hose numerous times, removed the cap and topped it off again. I've let it sit for a couple of hours now and the level hasn't gone down any. I will do a few heat cycles up to operating temp and see if I can add some more water after it cools down.
Thanks for the help guys.