I still think that power pwns cone area. Really there is alot more factors in the car that needs to be taken care of as well. But what would you rather do when you go to an spl comp? A dd 9915 with alot of power? Or 10 12" pheonix gold subs, with the same amount of power? See where I am getting at?
Honestly, physics disagrees with you.
Double cone area while holding power the same = 3db gain
Double power on a single driver = 3db gain
Double power & double cone area (hence, doubling your displacement) = 6db gain
But, one thing to consider is the power distribution as Kyle mentioned. When you just keep doubling power, only at very low power levels are you going to see a 3db gain. Once you start getting into relatively higher power levels, various power compressions are going to begin setting in......at some (not very high) point doubling power may only net a 2db gain, then a 1db gain......then no gain at all. There is a limit to the power game. In terms of increasing output, it's the least efficient method.
Here is Sundown's measurements from testing on one of their woofers;
17.7 watts @ 133.2 dB
33.9 watts @ 135.9 dB
66.5 watts @ 138.8 dB
120.4 watts @ 141.3 dB
236.7 watts @ 143.7 dB
448 watts @ 145.1 dB
765 watts @ 146.2 dB
999 watts @ 146.3 dB
1399 watts @ 147.2 dB
1886 watts @ 147.2 dB
Notice what happens. In the beginning, at very low power levels, the increase about matches theory perfectly. But what happens at higher power levels? The jump from 448w to 999w should have yielded a gain of 3.48db......yet they only gained 1.2db.
Not only will doubling your cone area net you a gain around 3db (all things equal), but depending on the power levels you may also suffer from
less power compression, again increasing the output relative to a single driver.
Now there are concessions to be made to the cone area argument. No, don't try to squeeze ten 18's in the hatch of a CRX. There simply isn't enough room, and you'd choke the hell out of the woofers.
But the best results will generally be had by
maximizing your cone area
first for the amount of room you have to work with. Especially considering the efficiencies you can gain from proper enclosure design, etc.
Throwing more power at the problem isn't the best answer.
EDIT: And your comparison of the DD and the PG's has no bearing on the discussion, as they are two different subs. Different factors at play, etc.