Sensitivity is a measure of acoustic output for a given input. It can be a power input, usually expressed as XdB@1w@1m, or a voltage sensitivity expressed as XdB@2.83v@1m. It is similar to, but not the same as efficiency, which is power out divided by power in.
What sensitivity means to us as musicians is that a more sensitive driver will usually sound louder. It takes twice as much power to get a 3dB increase in output, so a driver that is 3dB more sensitive will need only half as much power to sound as loud. Human hearing is logarithmic, though, so twice as much output doesn't mean twice as loud. In fact, it takes ten times as much power to sound twice as loud.
So with the Gamma and Kappa drivers, the Gamma is rated as being about 2dB more sensitive than the Kappa. 2dB is not a lot, and it won't necessarily sound louder, but you might have a bit more headroom. In the lowest octave, the Kappa will have 3dB more output, which should translate into a more solid bottom end.
The CB15 specs are very similar to the Gamma 15, so it would certainly work well. The CA15 doesn't look as good to me, but, of course, you might love it. Sound is very much a matter of personal opinion.