I have heard that water cools alot better than anti freeze, so if you use too much anti freeze = less heat transfer, but even so the thermostat should still be able to maintain a constant temp unless the radiator heat capacity is being just slightly exceeded for the normal operating temp
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.