We had the thermostat and water pump replaced with a timing belt change a couple thousand miles ago. Since then, the temperature gauge seems to run a bit hotter than I recall before it settles down to the "normal" place - about half way up. It's not up in that hotter spot for long - 10-15 minutes maybe - but it worries me. Could this be due to an aftermarket thermostat? If so, is it something to worry about?

If the thermostat installed was not a subaru thermostat, I'd recommend replacing it with a Subaru thermostat. Most aftermarket thermostats are not as deep, and have different responses to temperature change.

Also possibility of air trapped in the system, though I don't think that would give the symptom you are describing....