There is an old saying that I really think applies to this. Don't replace anything, upgrade everything. Going to a better muffler is always a plus from the stock, choked up one. And always leave your catholic convertor alone. That is why all the kits are called cat back systems. Yes, the extra effort is worth it, and your stock muffler is usually more expensive than Flowmaster, and you will love the sound difference. Sounds more beefy, instead of like a sewing machine.