For me, the best solution to controlling the coolant temp has been to use a 180 degree bypass type thermostat.  On a recent flight with 20 degree OAT, the coolant temperature, as measured in the water pump housing just below the thermostat, rapidly warmed up to 183 degrees for idle and taxi, peaked at 190 degrees during take off and climb, and stabilized at 185 degrees in cruise. 

One issue with the bypass thermostat was oscillation of the thermostat valve caused by the pressure difference across the bypass plate when the bypass was nearly closed.  This produced a clicking sound  in cruise and was felt as a shudder in the airframe.  Pulsing pressure was observed when connecting a gauge to the area between the water pump and the thermostat.  The pulses caused fatigue failure of the RX7 heater core located in the cockpit.  This may or may not occur depending on the spring constant of the particular thermostat.  After modification of the thermostat bypass plate as shown in the attached photo, no oscillation or pressure pulsing has been observed and no damage to the replacement heater core has occurred.  The area of the bypass plate was reduced by the modification but it was still large enough to completely block the bypass passage.

Another possibility is to block the bypass passage and use a non-bypass thermostat.  Although this eliminated the oscillation and pressure pulses, this may not be the best option since the overall flow rate of coolant through the engine will be reduced until the thermostat is fully open.  When the coolant flowing through the engine is unrestricted as is always the case with a bypass thermostat, the temperature rise through the engine at flight power levels is always close to 10 degrees due to the mixing of warm coolant from the bypass and the cool coolant from the radiator.  If the temperature cannot be completely controlled by the thermostat, the temperature will be seen to rise, but the temperature difference through the engine will still be close to 10 degrees.  With a non-bypass thermostat and the bypass passage blocked, the temperature difference between the coolant entering the engine block and the coolant exiting the block can be 60 degrees or more in cruise.

Eliminating the thermostat completely and blocking the bypass passage is another option.  In this case, the coolant temperature difference entering and exiting the engine will always be around 10 degrees or less, with the average temperature determined by the air side of the heat exchanger.  Controlling the temperature with a cockpit adjustable cowl exit flap was difficult, not completely effective, and significantly added to the pilot workload compared to using the bypass thermostat.

The few thermostat failures I have encountered were not sudden complete failures.  They resulted in the thermostat failing to completely close when cold.  I have not had a failure of the thermostat in the aircraft so far in about 350 hr of operation.