In general, any time you are moving more air in, you are gonna want to move more out as well. An engine is basically an air pump. The more air you move, the more power you will make (assuming fuel and spark are both increased appropriately). Usually the main restriction in this equation is the intake side of things. However because of the way turbos work, they generally benefit more from a slightly larger exhaust than than non-turbo motors.
That being said, I would imagine that a supercharged V6 would still benefit in the increase from 2.25" to 2.5" pipes. I wouldn't go any larger than that though, because 2.5" pipes will support a great deal of horsepower, probably more than the average blown V6 will ever make.
I'd say somewhere between 350-400 rwhp, the step up to 3" may be justified. Possibly a little lower for a turbo car.
