I wouldn't put a MM in my XXXT because with my novak SS5800 it's plenty fast already , too fast really. I can't thinck of the MM as usefull in a 1/10 truck , neither a novak 5.5 or similar. Not if you're racing anyway.

I know speed is what most rc guys are after but when it comes down to turning fast laps , less means more , less speed and more torque , that's what it's all about in my opinion

BTW I'm also located in Belgium ( Gent ) nice to see other Belgian people at this US forum.

fwiw...
 

I recently put a MM 6900 in my xxxt. It turned out to be "overkill" for sure!
I swapped out to a 5700 which is much better, but I still have to be very gentle on the trigger in loose dirt.
I geared way down to 13/90 before installing. The 6900 also ran too hot (almost 200 deg at normal timing!) whereas the 5700 is staying around 150 (low timing).
I'm getting ready to experiment with gearing up to see if that might reduce the initial torque (spinouts..)

Oh, I'm using 4200 stick packs.

3/28 - I changed the 13 pinion to a 17 and it seemed to reduce the wheel spin a lot! In fact, I think the truck liked the change overall (until I managed to break a front arm...)