It depends on the manufacturer really, I would not say LeoVince are superior. Brands like Akrapovic and Arrow are superior IMO and are usually my first choice. With some manufacturers a slip on will add 3HP to a 600RR and a full system will add another 3HP, these numbers are subject to manipulation in a lot of different ways. You have to prioritize what you are looking for, i.e. performance, sound, quality etc, and then decide.