The MAC header looks to be a much better flowing design than the stock, and I assume the muffler is not nearly as restrictive so it surely has some performance gains. There are very few options for exhausts.
I’m replacing it because it’s cheaper, I want to save my new stock header for a later date, and I’m not doing a stock restoration but rather a modified bike that you would have seen back in the day. The stock unit is a classic but to be frank I like the looks of this system. And if I wanted a fast bike I’d buy a BMW S1000RR.