[JesseK]

If you can't CAD the replacement, I'd recommend against it.

For a rough estimate of good speed vs torque, search CD-media's whitepapers for "JVN Calculator". Add 2 more CIM motors to the current gearboxes and figure out a better ratio for your drivers.

No matter what, realize that if you think you're fast -- there's always someone faster. If you think you can push -- someone else can push harder. Conform your design to your strategy, not to an errant edge condition. In other words: where on the field do you think extra speed would be useful EVERY match, and how much less agile will that make your robot in the cramped places on the field?