Won't help you much on gas milage especially if you're doing city driving most of the time.
I've had mine for the past 8,000 miles or so. Nice sound and a *little* noticable difference in performance but it needs to be cleaned from time to time. I thought (if I'm not mistaken) that Volant claims it should only be serviced once every 50,000 miles or so but after 8,000 miles of driving the throttle response became extremly slow. To the degree that I had to press the pedal and actually WAIT for the vehicle to move!
I removed it yesterday and it was BLACK (full of dust and smoke traces). Installed the stock filter to check the difference and WOW the Escalade is actually QUIET and SMOOTH. I miss the sound but I'll pass on that. I live in a desert where sand is everywhere and I'm actually enjoying it as quiet as it is.