In my opinion don't think you'll get better mpg with upgraded exhaust. If it did, you won't see the small difference due to you wanting to hear that exhaust with your right foot, lol. Sound and performance since you have a supercharger I can see that. As far as stainless steel, let's see what the exhaust looks like, maybe some "Mother's" polish would help for the time being. That stuff works wonders. Look at the difference in the first picture, left billets not polished, right are.
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