I have five MM escs and I run them all in 1/8 scale vehicles on 4S with everything from the Feigao 7XL to the 9XL. Heat has never been an issue, and since I added a couple of caps to the power line, the esc temps have gone down even more.

Just don't get to nuts with the gearing. If the MM is overgeared, it will let you know with cogging. Just as a point of reference, I run the 7XL in my buggy with a 12/46 to 14/46 gearing and haven't had any problems with heat on the esc.