a t-stat does not change the temp of the incoming air...where the concept of cooler/denser air will yield more hp. The t-stat decides when to open and allow the engine coolant to enter the radiator. So on a hot running engine, the lower temp t-stat will open earlier, but once it's open, the engine could still run just as hot as before. Hence, the point about seeing negligible hp increase due to lower t-stat.