If his sub has two 2-ohm voice coils, and he's wired them in parallel,
then he has an overall load of 1-ohm, not 1/2-ohm.  

With two 12" subs, you likely have more cone surface area than his
single 15" sub, but there are other factors in play, like the maximum
cone excursion and the sensitivity of the subwoofer.  I lke the JL
12W1, and have used them quite a bit, but they're not exactly
"powerhouses" when it comes to raw SPL.

As for the amps, I don't know the specs of his Crossfire, but he's
probably getting a pretty good boost in power by running it at 1-ohm
mono.  Your JL 250/1, on the other hand,  is designed to produce the
same 250W into any load from 1.5 - 4.0 Ohms, so you're not picking up
any extra watts by bridging the subs.  Depending on which version of
the 12W1 you have, your overall impedence is either four ohms or two
ohms.  For either version, the continuous power handling is 125W, so a
pair of them is a good match for a single JL 250/1 amp.  Regardless,
250W into a pair of 12W1's isn't going to blow anyone away at an SPL
contest.

If you want more SPL, I'd start off with a bigger amp.  As for the
speakers, you don't necessarily have to go with more or larger
speakers - you probably just need to find *different* speakers.  To
use an example, the JL 13W7 has more than triple the linear excursion
of  the 12W1 (29mm versus 8mm), and the power handling is 750W instead
of 125W.  Just one 12W7,  combined with a bigger amp, would put your
current setup to shame, and you wouldn't have to worry about making
room for bigger speakers.

Lastly, if your current setup uses a sealed box, you could look into a
ported box for more SPL.  For any particular sub. the recommended
ported box tends to be larger than the recommended sealed box, but if
you move to a single-sub setup, you may not need any additional space
to fit a ported box.