One aspect of power handling that has not been mentioned yet (unless I missed it) is the demand placed on the woofer when asked to play very deep bass. The deeper the bass, the more air that needs to be moved. With respect to the W18, for example, there is only so much displacement available with a 7" woofer. If there is a good deal of demand for very deep bass, the driver may reach its XMAX and bottom out.
The bottom line is that, in the real world, SPL capability will be determined, in part, by the amount of deep bass in the program material and the displacement capability of the woofers.
I hope that makes sense.
- Jim