I am not an expert on the whole metal temp thing, but I know that the fan cut-in temp and t-stat should be changed together. The fan can be reprogramed in the chip or tune.
With that said, IMHO, the engine should be at a proper operating temperature to be sure that all of the sensors operate correctly and so the engine has less friction. The statement regarding most cars run better when cooler, (I think) has to do with the intake air charge. I know that the intake air needs to be real cool. If the engine runs hot, then the intake gets hot. But if one could figure out the optimum engine temp and couple that with cool intake air, then that would be the best IMO.
As for my own car, KB sent me a 160 degree t-stat and the engine will not come up to operating temp. I will have to put in a hotter t-stat. I might go to a 180. If that does not work, then I will go back to stock. I live where it gets REAL cold and the 160 just does not cut it.