I had a cutout on my Neon long ago, right off the header, before the cat. The idle was awesomely rumbly and the sound at low RPM was insane. But, it was loud. Super loud. No idea on MPG. Car wasn't built for MPG at the time.
As for a cutout AFTER a cat? I could easily see that as fairly quiet. It's amazing how much muffling the cat can actually do. I've had several different types of "race" setups with different exhausts and after trying cat only, I can verify that a cat can muffle enough and, pending the vehicle, a car can be as quiet, or quieter, than some of those fart cans I've heard around town.
Now I'm just on stock exhaust, minus the short tube header I have on. Don't really miss anything fancier. It's just perfectly quiet... and cheaper.