You're probably going to notice about the same performance with either 40 or 50 series. I have a 40 series on mine and the sound is great. It gets a little droan at highway speeds, but at idle it has an awesome deep rumble, and when you get on it, it sounds great. 50 series is going to be a little quieter. You will see a little performance gain, but if you add an intake, you'll notice a good bit more.
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