I use the 678 w/o a thermostat. In the winter I block my cooler 2/3 of the way with a sheet of rubber. Otherwise, it will get down to a max temp of only 120F or so, which just seems to cold. In the summer with the TCC locked it usually runs ambient temp plus 80-90 degrees or so, so 170F is a typical summer temperature for it.The Hayden 698 has a 180degree thermostat according to their tech department.
I'm on the fence between the 698 and 678.
I have the OEM cooler installed and the factory thermostat pinned. 90+ ambient temp I have a pan temp of 190-195deg; it always matches the coolant temp.
I am aiming to get it around 175-180 tops.