[RC-Monster Mike]

Perhaps this person has the 16 cell c50 maxx motor. they were available in 3 winds - one for 12 cells, one for 14 cells and one for 16 cells. I don't doubt the 16 cell motor could handle 18 cells without trouble, but the 12 cell motor on 18 cells is asking for it. I assumed you were speaking of the 12 cell c50 (most common). Also, there are many factors that vary from one maxx to the next, as well as driving style, tire selection, gearing, weight, etc.. I would not recommend these high cell counts with the common c50 maxx motor. Just because someone has gotten away with it, doesn't mean it is ideal, or that it would work for everybody.
For example, I have run 18.5 volt Lipo packs through a 10 cell controller, using the controllers BEC for the last couple weeks, and have run 12-14 cells through the same controller for a couple years. The controller has held up fine so far, but I would never recommend this setup to a customer, because it "shouldn't" be used in this manner. It works and has worked, but you won't here me telling someone else to do it. There will always be exceptions to the "rules".