I run a Delkevic system. Stock head pipe still, but they make a cat/pre silencer delete S pipe and a wide variety of mufflers. Fit and finish is good/great, and they are light weight.
Plus the prices are good. Mine with a 14" carbon fiber muffler was $250ish USD shipped to a US shipping address.
For me its a bit louder than I'd like, but I kinda knew getting rid of both the cat and the pre silencer would probably do that.
The Leo Vince cat delete pipe has a resonator in it, and I suspect that would be a great way to go for a quieter sound.
fully open airbox
R6 rear shock