I don't think supercharging effects the gas mileage TOO much, but if you have a supercharger, your FOOT will cause the mileage to go down. LOL!
Yeah, usually you will have to run at least 92 octane or it'll ping. And pinging equals KAPOW!
A man may smile and bid you hail
Yet wish you to the devil;
But when a good dog wags his tail,
You know he's on the level.
If you're talking a mild amount of boost then you'd probabaly be fine on stock internals. Some 3.8L mustangs have run over 400 hp on the stock internals. Granted they made a few changes on the mustang 3.8Ls but nothing that would make a huge difference until 98 (heavier mains, neutral balanced crank).
I don't expect to get great gas milage with my SC when it's finished..... thats why I have my other bird