When I read this thread today another thread quickly came back to mind:
http://www.chiefdelphi.com/forums/sh...967+drivetrain
This should help give you all some inspiration on a drivetrain concept almost if not identical to yours, except it sound as though you want to mix things up a bit. Someone from 967 is WAY more qualified than myself to tell you how well this drive train can be. Also I am a big fan of what 1730 did with their ball caster/omni-wheel set up and I know an 8" wheel version of that is being talked about as a prototype drivetrain for our team next year.