You could use the Vex Shaft Encoders in FRC. According to the
Vex Shaft Encoder Manual, the Vex encoders can handle up to 1,020 rpm with 100 pulses per revolution. They cannot be geared directly off the CIM motors, but this is a decent speed. The sensor itself is a digital sensor, releasing a string of 0s and 1s. For every 01 pulse, that is 1/100 of a revolution.