Here's a vid of a single sub setup in that car sub forward ports to the driver's side. This box was 3ft net volume w/ two 4" ports tuned to 33hz. It hit a 143.5db @ 40hz with a Kicker cvr 15 powered by a Sundown saz-1500d at maybe half power.
I don't know if anyone has noticed but the port area in these setups is on the low side. It truly needs about 50% more port area. In the single 15" sub setup using the exact same box in the vid above I saw a 0.5db gain by adding a 3rd 4" port. The problem in this car though is it likes extremely low tuning and hits it's best #'s tuned down to 32-33hz. With six 4" ports for the two 15" setup they would need to be 30" long to keep the same tuning which is impossible in this car. We're throwing aroung new designs to get the port area where it needs to be because in theory there could be a full 1db gain just by giving it an adequate amount.