Hi Mike,
I have had aftermarket ice installs with 12" woofers and don't get me wrong, they sound magnificent - i had it primarily in my peugeot 106 Rallye and Gti so the base travels nicely through the tinned metal lol!
I just think that with the 8s, it's a different class of car (sorry Ska & Botang) and i don't like the the idea of putting aftermarket stuff. There are a number of other downsides like
(a) it takes up boot space
(b) bass travel into the cabin with less ease (i have the playstation and DVD controls in my ski-hatch you see)
(c) need for more amplifiers, pre-outs, cabling and the hassle this brings.
My theory is that although the difference will be marginal for the front passengers, the rear passengers will hear a noticable difference...but it is a theory.
The truth is that i'm one of those people that constantly needs a project and i've got an itch to do something that is unusual!
-Oli