I have to swap mine out too - the temperature isn't stable. Mostly it runs pretty cold. Might even be the temp gauge itself. I might try the hotter thermostat with some water wetter and a thinner mix than usual to see if I can pick up some mileage and cut parasitic drag.
Make sure you top the car off with distilled water! BTW, anyone have suggestions for antifreeze? I was going to try the Prestone Dex-Cool stuff. Is the BMW coolant better?