The CIMs are pretty tough when it comes to abuse, now that I think of it.
Last year, against my advice someone decided that a 4:1 ratio on the CIMs would be enough for the drivetrain

Well to their (the motors) credit, they did move the bot around, but after 7 minutes or so of driving around. You could smell some not so pleasant smells coming from the motors. So we added on some more sprockets and got it down to 13:1 or so.
Oh well, live and learn, then learn to use the datasheet. *sigh*