The stock exhaust is styled nicely. It might be heavy but it does look as good as a single tube can.
The slip ons seem to look similar to each other. With the side bags in place the rear part of the exhaust is hard to see anyway. The finish of the stock exhaust is one thing that has a pretty good benefit, it looks clean even after days of riding.
Form a styling perspective, Moto Guzzi did an excellent job IMO. As for performance, with a good map what more could you want?