A couple coments:

1. There is a cheap thermo and a more robust one available on the aftermarket. The cheap one will actually bend the top bracket over time, and fail in the partially open mode. This makes for pretty cold winter performance.
So, buy the more expensive one - can't remember the name right off.

2. Personally I would never want my engine to run below 80C, which is 176 F. That's not the factory intent, and borders on possible sludge formation, and possibility of piston scuff, not to mention longer warmup periods - not good.

3. Why would a cooler thermo be better in 120 degree heat? The only time it would matter would be during initial warm up, where it would heat up to operating temp just a bit slower. But once it warmed up in that kinda heat, any thermo would be wide open and the radiator capacity would control the operating temp.
Now, if it were pretty cool, the system mifght be able to keep the temp down to the opening point of a cooler thermo, but the 87 thermo runs around 83 in these circumstances, so what added good would 79 or less do - see #2 above.

DG