I would bet you at 10 to 1 odds that the STi in question also has a vent to atmosphere blow off valve. That is the source of the back fire. Not having a cat makes it much louder when it backfires, and in many cases lets the actual flames shoot out the tail pipe when the car backfires.As far exhaust components, just a hks hi power and downpipe. what about back firing? Ive heard some STIs with a cat delete and their shit back fired left and right.