I listened to the Alley Kats on the Wing in person at Americade this past week.
I then purchased and had them installed on my Wing. I rode the bike about 1200 miles back home. I did not note in change in fuel consumption as compared to before. I did not check the mileage before and later except that the Wing traveling with me was adding about the same amount of fuel at each stop both going up and returning. I have not noticed any change in power nor did I expect any change.
My evaluation of the produced sounds are that they are really nice at idle and while playing with the throttle in neutral. While moving all three of my traveling bike buddies said that they could not hear any differences unless they were immediately behind or next to me and they all liked what they heard. But for me, at 50 to 90 MPH, the generated loud low frequency sound was to much for a long trip. I will probably reinstall my stocks before my next rode trip. The supplied warranty excludes any guarantee "to produce or cancel any particular tone, note or resonance, as sound preference is purely subjective".
As the warranty implies, sound preferences are a personal issue and are subject to our personal evaluations. For me this could be an expensive lesson.