We're using the Shimo chassis from the conversion kit and happy with it, but haven't tried the others.
Re: the negative diff, where are you?
The new mid. shaft looks to be out late December according to AMG's broken English.
Also, having a trawl through the Kyosho site, it appears there are a couple of new spurs available.
1st. Spur Gear 0.8M/59T) VZ113-59 \ 400_
2nd. Spur Gear 0.8M/57T) VZ114-55 \ 750
. . . be carefull, here. It looks like there's a typo in the part # for the 2nd. Spur. I would think it should be VZ114-57
if it's to follow their usual pattern.