I know that Jaguar offers two thermostats, 82C and 88C. There is a Stant thermostat that's been recommended here, 45398, which is a 82C thermostat. I accidentally bought 45399, which is a 90C thermostat. I can get the lower temperature ones, but how much difference does it make?

 

It doesn’t….if you don’t mind the engine running at that temp, which I believe is fine.

The thermostat maximizes the coolant flow at a certain temperature. In your case 90C, or 194F.

Does that mean your car can’t get to 200F? No. If there’s something restricting the coolant’s flow, it won’t matter if you have a 90c, 88c, 82c or 72c thermostat. It’s gonna overheat.

The thermostat you’ve selected can no longer retard the cooling of the engine at 90C. So the engine will get to 90C and stay there, if everything is running nominally. If there’s a problem, changing thermostats won’t do a damn thing.