Running at 79c doesn't make a whole lot of difference to be honest. I'm getting the same ~30mpg on the highway with a 79c thermostat that I did with a 87c thermostat. Last tank was 32.5mpg. I did notice that it pings less on 87 octane under high load than it did with the 87c. Mercedes used a hot thermostat to reduce emissions, not for fuel economy. I would keep the cold thermostat.

__________________
CENSORED due to not family friendly words