I hear they are pretty solid. I'd also look into a Mini Inferno 09 and maybe a Trinity Next. All those come with pretty solid drivetrains. I think 70-80% of the VTX parts are compatible with the Kyosho MI-09.
A center diff/motor mount would be nice, but there are tons of existing mounts you can use like the MQ mount, 1/12th scale mounts, etc... you just need to know how much space you're working with and what size motor you want.
MQ mount:
http://www3.towerhobbies.com/cgi-bin...?&I=LXJAE3&P=7
AE 12LC mount:
http://www3.towerhobbies.com/cgi-bin...?&I=LX2822&P=7
I'd suggest a 400 sized (28mm diameter) BL motor for the 1/16th projects. Of course, feigao makes some great 400 motors for very cheap. The 380-8T (~4700Kv) is very potent on either 2s or 3s lipos. I also like the Kontronik, Permax, Great planes ammo, etc... All are just as efficient as Neu motors, with 2-pole designs so that cogging is almost non-existant.
I'd also use a MM for the esc. the 1/18th scale esc's just don't take enough amps for a truly good conversion.
Anyways, the MM is only a few more $$$, so its an easily justifiable upgrade.