I am pretty sure that is the stock seat. I have never seen any aftermarket that looked exactly like the stock.
I put the Kaw gel seat on my 900. It was much better for my wife because the pillion area is several inches wider, but I didn't fell much difference.
Sorry, I don't have a better pic of the gel seat.
I know a lot of people like Mustang. But, I cannot comment on them; as I have never ridden on one.