I have been running my Emax with MIP CVDs and stock diffs and havent had any problems yet. I however will NEVER buy MIP CVD again. That dumb pin they use is a pain in the butt. I ended up welding the pin holes closed so they wouldnt fall out.
I have the 8xl also on 4s lipos. Not sure about the gearing I run a RRP slipper 66t with 16t pinion and it seems like the right combo.

I would assume 66/16 is 4.125

so a 51t/20 is 2.55

try 51t/13 is 3.923
or 51/12 is 4.25.

Not sure if I got any of this correct but it seems correct to me.