wow, thats a ton for a 3" coil..
The old 4hp (10.1" slug) pulls BL^2/Re of over ~500 with a short ~2.2" long AL coil - (8 layer flat) switch it to copper and it would get 800 N^2/W :P but that's a 4:" coil for ya.
TSP's can be inaccurate but they don't generally get the final Q or sensitivity wrong, if they do get the BL wrong it will be offset by the moving mass. In other words, the 12" Might be more correct than your 15 or the other way around. If the BL is too high, then the moving mass will also be too high, and if its too low the moving mass will be too low too. The way to confirm is the cut the softparts out, cut about half the surround and spider and measure the moving mass. Then you can back out the real BL by keeping the Qes constant. Broken in really wont make a difference if you use enough power. you should use at least 2.8 volts for woofers.