we did this a few years back with dragpipes on a 900. the headers weren't super pro but they were 1.5 inch stepped to 1 7/8's so they were similar. it had a biggun can on the back very similar to the super pro setup. then we added a few different size megaphones and had to jet up sizeably. and gained 3-4 rwhp. I don't see why it wouldn't work like that with the super pro.
I think the megaphone would be a nice gain. burns stainless sells nice ones. then if you wanted to trailride you could always swap back the muzzy muffler and easily trailride.