You'll need a downpipe with a high flow cat. It the boost quicker, and you'll notice a difference. The reason you go stage II with a downpipe, is to prevent boost spiking with the added flow. I'm happy with my tune. I'm not sure if it's worth $500 for the tune and however much you're going to spend on the downpipe. To save money, you should get a custom downpipe from a reputable exhaust shop, at least 2.5", preferably 3".