I added a gear diff to my 13.5 car over the weekend. I'm also running a slipper eliminator. It is very smooth and predictable. Since we run sugared bluegroove there is enough traction for the eliminator, even with a 13.5. The car was smooth as butter with 5k in the diff. I would do the same thing for my 17.5 car(s) if they made pucks for gear diffs like they do for the Kyosho cars. Bottom line is that the gear diff was worth at least a tenth or two in my average lap time, meaning that it did not make my fastest laps faster, rather, it made my lap times more consistent and helped me make less mistakes.
I have to say, working on the B5M is much easier than working on the RB6M. I think that in this case the way AE designed a specific MM design is very apparent. I was able to get the diff swapped in a few minutes. There are a few things that really stand out, small things, like the fact that the tranny uses 4 screws that are all the same.
I'll also add that AE's parts quality has really improved. I definitely can't complain about any of my B5s wearing parts fast. I've done a lot of racing and practice lately and the outdrives on my non-pucks cars are all still near perfect after a couple months.