How much of a performance difference would it make?
It would make very little difference in performance. I would put a spinnaker on it before swapping out the mainsail. But really, if you're looking for a boost in performance, you'd be better off saving your money, selling the Getaway, and getting a higher end boat.
What you're talking about doing is akin to souping up the engine on a mini-van. Yea, it might give you a little boost in performance, but it's still a mini-van - better off just saving up and buying the Porsche.