The C rating is only the maximum amp draw that they can handle. It does not say anything about how efficient they are at delivering that power.
Great point that I neglected to mention in my earlier post. The better/newer batts will hold a higher voltage at the higher end of the amp draw limit. amps are only half of the power equation (P=V*I) Power = Volts*amps. I am sure it is similar with lipos, but I know with my nimh 1.2 that the cheap ones can drop to ~1.10 V at 15 amps and the good ones hold ~1.20 V at 15 amps. You can calculate watts if you can measure amps and volts when the circuit is under load.