Yeah, I have never heard a set, but am very sceptical. I wouldn't spend the money on them myself. If they were as efficient as regular subs, all designs would be heading in this direction. There is always a way to make regular speakers fit. I am putting a set of Boston Pro 10's under the seat of my Dodge Quad cab. And I know those Fords are easy to raise the seat to give you more room if you need.
But, if you do end up installing these speakers, do a write up here, and let everyone know how they sound.