The problem is not the speaker; the problem is the amplifier. It's only a 100 watt amp, and it's integrated into the box. It doesn't matter what speaker you put in there; it'll still be driven by the 100 watt amp. If you find a speaker that's 6dB more efficient than the current one then it'll sound twice as loud, but that'll be difficult given the limited amount of airspace you have to work with.
If you want a molded box, there are several to choose from. Integrity Concepts makes enclosures for 10's and 8's on either the driver's or passenger's side. I believe they're a forum vendor. Obviously, you'll need to provide a sub and amp.
http://www.integrityconcepts.net/sci...-products.html
OEM Audio+ makes a very nice 8' sub, but it's integrated with their full sound system. Which is also very nice. They're also a forum vendor.
http://oemaudioplus.com/subaru/brz-2/
JL has a loaded enclosure, made with typical JL quality and prices.
http://www.crutchfield.com/p_1369456...tealthbox.html
MTX makes both a loaded/powered and unloaded enclosure.
https://www.mtx.com/sfrs13fb10a-tn-s...ofer-enclosure
https://www.mtx.com/sfrs13fb10u-scio...ofer-enclosure
There are probably a few others out there as well.