Of the five Mamba Max escs that I have, two will run 5S well. The other three just sit and do nothing when I plug in a 5S pack.

Considering that the MM is half the price of the Quark, I'd just setup for a 4S system and run the MM with the 7 or 8XL. Just make sure that the battery is up to the task. What does your friend have in his truck, as far as the motor goes? Is he running the BK 9920?