haha, sweet

I sell RE audio, I just wish their amps were a little more durable, I get tired of replacing them.

Pics or it didn't happen.

Two 15's are feasible..I've done it..bad idea..(too much) :eyetwitch
-They'll have to have seperate boxes, otherwise you wont get them in there. (unless you go for the fiberglass build with reverse mounted subwoofer route)
-You'll have to remove the rear seats along with the support frames to get the boxes into the boot)

Few words of advice
Dont buy 15's jus because theyre 15's.. theyre not all created equal.
Remember to take the weight factor into account..balance, handling WILL change.

(window shatter wont be a problem, the rx8's not air tight enough to alow the compression necessary) trunk will need sound deadening.

Now I have ONE 15..and its still overkill.

Two 12"s in a vented box will probably be just as loud as two 15"s in a sealed box. The 12" setup being easier to install. I little fiberglass work and some 4" diameter PVC if you plan on doing a low tune (long port). If you tune high, it might be easier/look nicer with a rectangular port.

I had two 15" L7s with 3kWrms in my firebird. I know loud. I had one 12" in a vented box in my 8. Bumped DAMN well. I now have two 10"s in a sealed box. About the same output as the single 12" vented. Two 12"s vented should be enough even for a bass head.