No, you don't need a catalytic converter (unless you livw in a commie state where your vehicle gets emissions tested). The catalytic converter simply converts alleged "Green-House Gasses to supposedly not-as-bad" Green House Gasses. Think of your vehicle's performance this way: if it can't get out, it can't get in. Or in other words, the more restricted your exhaust system, the less air intake to combust fuel = poorer performance.