Speed 900's at 10:1 sounds OK, maybe a little fast on those size wheels but will check that later.
With regards to not needing the extra power - drills are good, and they drive fast but they just dont have the grunt that sets a normal pusher apart from things like satanix and 540 that can slam people around really well. Ploughbot has got other robots out the arena by slamming them into the wall. Powerful motor's do make a big difference.

Reliability is the key though - design a solid gearbox for them and the correct ratios and you will be fine.