talk to some of the box designers in the enclosure section of this forum. hang out for a while, find out who knows what they're doing, dont just go with the first joe to say he can make you a box. make sure its properly built, airtight and all. assuming you car isnt one of those freaks that just doesn't work with bass, with that much power on 15s you'll be good.
dont know off the top of my head what the excursion on the 9500s is but its not all that relevant. a pair of 9515s would get definitely do the trick but i dont know that i'd feel as comfortable running 4k daily on one as on those other subs i listed. i used to run a single 9518g on a kicker 2500, so i've played with one, although not on that much juice. my audioque hdc3s are very similar to the 9500s but a lot cheaper, you might look into those, but again, thats a lot of power for one. if you know what you're doing and are careful with the power, shouldn't be an issue. this weekend i'm installing an amp that does roughly 5500w rms on the pair to see how they like it and i'm not worried, they never even got stinky on the memphis 4k.