Yes, the one 15" L7 will SLAM... I have mine in a 4.5 ft^3 box tuned to 35hz in a Chevy Blazer.. It is recieving 919 watts.. Playing a 50hz test tone it will make the tin roof of our town's Sonic rattle back and forth. Also makes the grease pit there vibrate.. As for flexing windows, NOOOOOOOOOO! You can feel some vibration on windows, and possibly hear them rattle some, but you will definitely not flex them.. I have a good bit of roof flex in my Blazer, but thats it for flexing. Mine can be heard from several blocks away..
Thats my experience with it.. Like others said, yours will be different since it is in a trunk.