I'm guessing you wouldn't see much power increase going from a grumman muffler to the powerflow, but fit and finish of the powerflow product is much better than the typical OE exhausts. Whether it is worth the price premium or not is something you'll have to decide.
If you decide to stick with what you have, I'd go for an overhauled or new muffler. The material thins as the exhaust system ages and while a repair might be more economical at the moment it may cost you more money and inconvenience in the long run.