From what I've been told by experts, you really start to hit the point of diminishing returns just adding more material to the motor. Most of these super-duper subs must sacrifice a lot of efficiency just by using enormous coils (so that people can brag about how much power they can take).
Think about it, if another 15" woofer is moving 36mm peak to peak on 1500w and yours is moving 36mm peak to peak with 3000W, the only reason you're running 3000W is so that you can tell your buddies that you are running 3000W.
Anyway, bigger coil requires not only a heavier coil, but a wider gap. Magnetic force requires some pretty hairy math, but the distance has an exponential effect on magnetic force... you can see this for yourself by taking two small magnets and moving them together slowly or trying to hold them apart from each other just a hair vs an inch apart.
That all said, most of these "team" subs are a pretty fair tradeoff of efficiency for power handling though much of the power handling is only for the sake of power handling itself, and if you're limited to a single 12 I'd say any of them would be a good solution for what you're trying to accomplish.