We used a single mini CIM on each transmission last year give us more space for totes in our robots. The min CIMs gave us more distance between our transmissions, and saved us a little weight. The two mini CIM drivetrain was still sufficient to push our loaded robot around the carpet.
In addition, Vex has a fun little plot for the CIM and mini CIM motors on their website that shows a 3 minute peak power test. Looking at the two plots side-by-side, you can clearly see that the mini CIM has better peak power output characteristics (especially in the first 20 seconds of the test).
http://motors.vex.com/cim-motor
http://motors.vex.com/mini-cim-motor