i heard someone with a W7 13.5 in a JL box, not the HO, but i guess it was the pro wedge maybe. anyway i would put my 05 XXX 12 in a 3 cuft slot ported box tuned to about 32 hz against it anyday.
3rd Gen mag 12 IMO outdoes it. W7 has a sound that only a few really like and at high volumes does tend to become loose and not as tight. Output wise they are neck and neck. I've done a drop in test with both the Mag and 12W7 and the Mag did better SQ wise and output was within 1db of each other both flip flopping who's was better. 13W7 had more output but SQ went to the Mag. I work now at a shop that sells JL and even the head installer likes the Mag more than the W7.
Your box is not going to make that much of a difference if its the JL HO box, its just made to the W7 specs. Cause you can make a box to match SI's( or hell call Nick and ask him) specs and it'll be just as good.