I know Scott hasn't updated the Fi site, they are listed on his Home Theater Shack forum though if you're still curious. They come in more car-audio friendly impedances and I believe they've got a higher Fs which would help a bit in a trunk due to the massive cabin gain. Not trying to talk you out of things, just trying to open up some other opportunities! Also check out the Cult of the Infinitely Baffled, they provide some general direction on volumes needed for certain T/S parameters. Just beware that it's a home-audio only site, you'll get ignored or flamed for asking car questions there. I'd venture to say that "ideally" 2 15s would be too much/max allowable for a trunk, but "ideally" is often different than the real world. I wouldn't expect you to be able to squeeze more than that in without compromising your performance based on it being trunk-baffled, I've only thought about going this route tentatively though so no real-life experience.