I've not personally run my new X11 motors in my XX4 yet.
But Chris Bowater borrowed my car at the Marston Indoor Meeting and I dropped in the X11 5.5t, geared on 17/84 and it ran very well indeed.
Well enough to convince Chris to make a brushless purchase the following day.
I think I'll be dropping it back to 16/84 when I run it next week.