Most, if not all trans fluids are tested and designed to run at 175 degrees. Of course failures increase with temp ie 195 degrees and you will loose trans at 50,000 instead of 100,000 miles. Here is a site of interest
http://au.geocities.com/ozbrick850/at-mortality.html I have the thermostat you mention, but it is from BM racing and is for transmissions. Hope to have it in soon--kinda getting cold here in northern PA.