I've heard the Stroker 18" and the 15". Both were in hatchbacks and the 18" didn't make me gulp, it made me gasp for air. One powerful sub there (lets not forget that when the guy played this sub in his car for me that he did not have any rap so he tried to find a station that had rap, we ended up listening to Michael Jackson which i don't feel was the best display of bass). The 15" pretty much does the same thing, it'll pound you hard enough that you don't dare turn the volume up too high.
That's my experience. But that was years ago and am now looking for that kind of bass without the space hungry needs the Stroker requires for a box. So I'm going for the Stereo Integrity Magnum D2, two 12's in a hatchback. We'll see how they stack up.