That's not a realistic example, and it implies an incorrect conclusion. First, you cant simply say two subs with such vastly different power handling abilities have the same ts specs, that's extremely unlikely. Second, the enclosure size/type/alignment plays a huge role in over all SPL output. Third, xmax isn't the end all be all determining factor for output, especially in vented enclosures. Lastly, in a vented situation, the enclosure is most efficient at or very near tuning, and cone excursion is minimized... so power handling becomes a bigger factor in peak SPL output than xmax.