Since I'm trying to heat a large square footage this winter with as little supplemental propane as possible, I usually run my insert about 400-480* (stove temp). My sense is that this is on the higher end of what people here run in terms of stove temps. Curious if there was a certain maximum I should never exceed, I emailed this question to Hitzer, describing where on the stove I was measuring temp. Dean at Hitzer emailed back saying that he really didn't know since they never measured a temp at that exact spot
but that I should burn the stove at the temperature that gives me the heat I desire in the home "over time not just all at once". He also said that "the hotter you burn the insert the more it can affect the life of some of the components somewhat".
So, it appears there is no absolute maximum as far as the equipment is concerned. When outside temps drop again I may try pushing it a little more (550-600*), but i suspect that there are diminishing returns in terms of heat output into the home as you push stove temps up, with more heat wasted up the chimney and more fatigue on stove components.