[apalrd]

In using a CIM, you are looking for power (yes?)
If you run the motor at 18v, you can use a Banebots 775-18.
My math says that, at peak power at 18v, it has somewhere approaching 500w. A CIM at 12v peak power does 350 or so. (this seems wrong to me, so I checked it, and it came out to be the same thing).

So, you want a power-dense motor, not necessarily the most powerful, (at 12v) the most power-dense KOP motor is the 2011 FP motor, with 290 watts at the size of a RS550 (smaller than a CIM or 775).

Past power, it's really just gearing to make it have the characteristics you want. As earlier posters said, the power to fire the shot is actually determined by the spring, but the motor must recoil the spring, which requires work, so a more powerful motor can do the required work faster (meaning it can fire faster).