hmmm sounds like a question for glass or jonathan. I would say it has alot to do with sound deadening in the vehicle itself. all cars are different.now has it ever been done,i dunno but im sure someone does.
I'd say it's possible, i've heard of ppl hitting the mid 135 with one 10'' brahma powered with 600wrms. But like Napalm said, there are alot of factors, sound deadening, the box....
I saw a single bandpass 10" sub hit 144dB to take an event record in a Kicker showcar (GM F-body) so I'm sure you could pull it off. Particularly if SPL is your only goal and you're not worried about how it sounds.
i'd bet money that you coud do 140dB w/ an IDmax 12".
i have 1 10" brahma in a 3.6 cuft enclosure w/ a bp1200.1 and i'm doing 138.6dB legal. and the box still sound pretty good. so, i'm pretty sure you can make that idmax 12 get 140dB. it all depends on your install and your car.