Thermostats are a touchy subject apparently. Most say to definitely not put one in, but in 50 years plus of driving everything from cars to heavy machinery, I have yet to have one fail.  Now I find myself in the same position as yourself with a rotary that is too cool. there  must be only the two of us in the world.  Yes I also have too much cooling drag but am now restricting the air inlet to try and get the cooling right for my summer temps, and reduce the cooling drag. Today I cut the inlet back to 80 square inches or 18% / 20% of total exchanger area. It appears that I may have to restrict further yet!  I ran it on the ground at 4000 Rpm expecting it to overheat, stationery, but the water only got to 180 degrees and the oil to 165 degrees( 100 degrees OAT).  My cowl flaps only make about 8 degrees difference.  Did not fly as it is possible that my computer A may have died. Ran everything on computer B. My aim is to get the cooling right for the 115 degrees that is possible in summer here by restricting the air to the heat exchangers, possibly take off the cowl flaps and put a thermostat in the line next to the water pump for winter temps.  I may also have a second setting for the inlet restriction to restrict the air more for winter as well, for is only 5 screws to change the position of the air inlet.  My aim is this. To get normal cooling as close as possible to perfect without the need for a thermostat, then put one in to accelerate  the motors initial heating up from cold and to maintain that temp at altitude without the need for cowl flaps, even though I already have them.   Neil.