I'm running dual 8s in the rear deck. I didn't want to give up any trunk space, and I'm pleased with the results. I'd say they're about equivalent to an average 10 in a sealed box in the trunk less a bit of deep bass extension.
I'd probably go with some 8" Kicker Comp's if I had to choose again. I used a single Kicker 8" Free-Air (similar to the Comp's but no longer produced) with good results too.
But, a single 10/12 in a sealed box in the trunk would be much better for SQ (if you're willing to give up some trunk space).
