The cat is what restricts most the air flow,the stock mid pipes slightly narrower than than the megan mid pipe. I got from the header back hks exhaust you do notice a difference but if your like me you just keep wanting more. depending on what your goal is for power obviously sc or turbo the way to go, I never nationally got the fit for power but compared to my old car a 2000 kia saphie it seem awesome. The catless down pipe/test pipe will give you a cel the o2 fix might or might not work didn't on mine but flashpro fixed that.