Just put one in for my trip to the Glen. Wow what a rush!
I can't tell if it made the car faster, but it was very loud.
Loud enough to make NSX and ZO6 owners come over to see and compliment the shriek.

As for gas mileage, on the track I blew 3/4 of a tank to do 144km. Usually I do 400km on a full tank. Very Sick!