A highflow cat is an important part of a performance exhaust system. The exhaust works together as a package, so looking at the gains of an individual part is pretty much worthless.

If you slap on a highflow cat on a stock catback/manifold, you probably won't see much difference. When combined with properly matted catback and header it will make a difference.

It won't puff black smoke, gas mileage shold be the same.