that isn't right that shouldn't make a a difference in drivability if any thing it should increase you hp. sounds you shoud put your stock back on and see what happens and compare. i have the same car but i only have a apexi n1 no haeder but form what i know dx motor d16y7(non vtech) the cat and exhaust manifold are one piece and ex models d16y8 (vtech) are seperate maybe try running a cat and make sure the o2 sensor are hooked up correctly... and if your DIY double check your work.