I didn't have any luck firing it off the rear window, but I have half the cone area you're going to have. Firing it off the rear window made the rear window flex more than the front window, and you already know how much our cars flex.
Everything that I've done in this car has not been successful pointed up. I had 15s in the rear seat port and sub facing up and it fell on its face. I had them forward and they were loud. I had 2 12s sealed facing up in the trunk and they weren't as loud as if they were loading off the trunk lid.
In addition, unless you have a plan on fitting 10 cubes in the trunk, I just don't see it happening. I've got 4.6 cubes in my trunk, and I don't see 10 cubes being possible with the amount of batteries you want to put in your car, unless you're getting rid of the back seats/ floorboard. Even still, if you can fit 10 cubes in the trunk, .8 of that will be displacement. 9.2/ 4 = 2.3 cubes, which puts you in the same situation as me... not enough airspace for your 15s in a ported box. I'm running M3 15s which are 300 rms, 2.5'' coils... which will be similar to what you plan on running.
That being said, I think you'd be much better off trying 3 or 4 12s or 3 15s ported rather than 4 15s in our trunk. You need 13 cubes for 4 15s. That'll give you about 3 cubes per 15. Subs fall on their face without the proper airspace.
My next setup will be bandpass, 2 12s... Port facing forward, sealed off from the trunk. 6-7 cubes, if all plans work out 7500 RMS total.