A 6 WD with a dropped center does its job. We made a 6 WD with a dropped center using a standard toughbox, and that thing was crazy. It was quick, zippy, and the torque was crazy. As per a 8WD, I think that the were dominant this year because teams didn't want to bottom out on top of the bump. But usually, you'll see 6 WD.
As for gear ratios, read
this post before you think of something crazy. I personally agree with this logic. One can always find a nice balance.