I am also leaning towards mounting the TTi 2.5" X-pipe assembly (muffler delete pipes) with the aftermarket SS 2.5" tips, and then starting in the back with as big of a round MF muffler that will fit. From the rear of the car, a 6" would fit but since they are Center/Center outlets, the 6" muffler will set down lower and will be seen easier from the rear of the car. A 5" round muffler will fit much better with less muffler being seen from the rear but there is only one selection of 5" (12866 2.5" = 535cfm). If I decide to go this route, I will look at the 5" round x 14" long body beside the gas tank and then see how loud it is with these mufflers. Then if it is too loud, then mount the 4x9x14's under the seat.