The cabinet and ports affect the tuning of the Sub, not the output, as I understand things. The VTF 3 and VTF15 subs use the same Amp and Driver, according to Hsu's site.
This is not to say that there will not be some subtle differences in output, as extension will (IIRC) lessen the output a little, but in this case, the numbers show that it is still quite a monster.
Anything to clarify, Shady?