This is one of those things where some "old hot rodding" advice that might have meant something due to completely different engine parameters might have yielded gains, but no longer. Do you really think the engineers who specialize in selecting the coolant temps best for the engine went out of their way to select something inappropriate? There is another consideration- using too low a temp may cause the ECU to determine the engine isn't completely warmed up. The ECU uses coolant temp as a modifier to injector duty cycle, resulting in a richer mixture at all times. In the worst case, you may not enter "closed loop" mode. Meaning you will get much worse MPG.
