I think you may be going a little overkill with the motors there.
If you want something to go fast (as in, high speed, not high acceleration) then you just need enough power to counteract the drag you would see at that speed. Getting rid of drag is just as good as adding power. I would suggest focusing more on a small profile robot with a gearing system that lets you go through a wide range of ratios. With the size of the components, I don't think you'd need more than 1 CIM to power it if it was in an aerodynamic shell.
Also, you should get rid of that 6wd tank style steering system... It works good when you are driving a robot around small spaces on an FRC field, but you won't be able to control a fast moving robot with it very well. Cars use Ackermann style steering because it's easy to control when you're driving down the road.