don't have any more info on the behind the seat subs, but the setup looks pretty straight forward, but as pablito was saying, 8" subs in the doors like this would work.
the owner of this car is "AlpharettaRK" on the rennlist.com, you can try e-mailing him though his rennlist account for more info. looks like a custom setup to me; any good installer could fab up something similar.
my only concern would be if the doors could handle such vibration without devoloping squeaks and rattles . . . especially with the subwoofer's weight being thrown around by closing the door too hard. the enclosure would have to be very sturdy.
ideally, i'd look at a quality 3-way component set up with the tweets and 4"s in the dash and the 6.5"s in the doors. and a couple of 8" subs behind the seats like the previous picture.
moreover, you can probably design boxes for an 8" sub that are situated lower and partially under the seat with the sub at more of an angle, whereas those flattened 12"s would be upright and probably interfere more with how far back the seat could go. i'd get in there and take some measurements if i had a boxster . . . soon enough.
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