My thermostat is sometimes a little slow opening, especially if the the car has not been used for a while. Your thermostat is new and I would not be surprised if it was a little sticky to begin with.
The worst this usually exibits is a rise to 90 before it pings open, drops to 70/75 and settles at 80.
However I did some work on the heater this weekend and the first run out the temprature got to nearly 100 (and me having kittens) before the thermostat opened. This was due some air in the heater making its way round to the thermostat. A quick top up of water and all appears to be well again.
Nick