That's gonna vary a lot based on driving conditions and style. If MDS doesn't turn on often for you, you're not going to notice much of a change at all. But if it's on for 90% of your driving, you'll probably take a significant hit.

I'd say, get the programmer first since it's useful regardless of the exhaust. Disable MDS and drive around for a month or so to see how the gas mileage is really affected.