Andy, I have not noticed any change in gas mileage. The cans are still in new condition after 6 months of having them, and about 2000 miles of use. I do park it in a garage, so it does not get constant exposure to the weather.
And the pipes fit snuggly, there are no leaks or rattles. It just has a gap that is a bit wider than I expected at the junction.
They look great, and I am still very happy with them.
Evotech Tail Eliminator
14T front sprocket