Im beginning to think I should step up to a 15" Titanic based on the fact I just measured my living room and its around 4200cu.ft and open to my kitchen which is all of 2500 itself.
Is the primary listening position in kitchen?
How loud do you want to listen?
Room dimensions will determine when and how much room gain you get (along with construction methods). Distance from the sub to the primary listening position will be the major determinant of how much output you can expect.
Real world example. My comfortably loud listening level is around -15 below Reference. I have a single 315HO in 1 ft^3 sealed powered by a BASH300 in my living room system. For relevancy, living room is 22'x20'x9' with a 2 story foyer-open stair case-catwalk where one wall should be...easily something north of 8,000 ft^3 the last time I tried to calculate it. My primary listening distance to the sub is 14', according to Audyssey/tape measure. Even with the sub placed mid-wall (no corner reinforcement) and all of that acoustic space, that little sub is capable of going louder than my comfort level...which results in peaks around 100dB on demo level soundtracks. Now my idea of loud might be your background noise....YMMV and all those other standard disclaimers.
That said, the T15 will give you 3-4dB more across the board than a T12 from the same power and it'll also handle more power for an even greater difference in max potential output. The downside is you're looking at a 6 ft^3 box minimum, IMO...and 8 ft^3 would be much better.
-Brent
