It's the shape of the cats that creates the clearance problem. The round ones like SLP or new Bassanis probably drop 1/4-1/2" lower than an oval shaped/flat bottom ones like Magnaflow. For that reason, I would go Magnaflow. The actually X-pipe crossover section does not hang down much lower than stock. I'm lowered 1.75" with coilovers, and the only thing I ever hit was the mufflers a little - they are the lowest spot -- about 3" from muffler bottom to ground.
