Are you sure there is even a thermostat in there now? When I bought my SDL it would run hot in the city, too cool freeway... pulled the housing, there was no t-stat to be found.
The car had a bad fan clutch so the PO just pulled the stat as a halfassed fix. No shock really as the heater core was bypassed too for some reason.
Anyway, I have never had my cars run around 90°C constantly. Even during the hell we had last summer. They may creep into the upper 80s with the A/C on full load. NOW, I do have all cooling system updates available depending on the drivetrain. Late fans, different pulleys, new radiators, correct thermostat, etc.
You would be shocked how much cooler the car will run with simply a new radiator along with a clean A/C condenser.