An 84-degree thermostat is designed to open at 84 degrees in a normal operating environment, meaning in a pressurized system. Additionally there is a margin of acceptable error in the calibration. The dash gauge will always read different than what the thermostat is rated at since the coolant temperature sensor is not located at the thermostat housing.
If you think MBZ's are crap in comparison to a BMW 850, you are in for a big surprise. I can tell you first hand that the 8 series BMW is VERY temperamental. I speak from experience from owning both an 850 & 840. Had I known you wanted an 8 series a few months ago I would have gladly sold you the car at a deep discount.
PS-The 850's are notorious for running hot, ask your BMW dealer or check out some of the BMW tech sites if you doubt me.