Have a stainless Yoshimura R-1 (slip on)because of a good deal thru E-Bay. The quality is great (as expected) but was even more impressed
when I took it apart for mods. The stockYosh is slightly louder at idle and quite a bit louder at full throttle (but not WAY louder).

 

I've got a Jardine RT-1 street baffle aluminum slip on and like it. sounds a little "fluffy" at idle but really comes alive once you get going,over 7k it just sounds awesome.
My friend has a '06 600RR and has the Yoshi RS5 carbon cone ,sopunds great.

 

Either the Yosh RS-5 or the Jardine RT-1 Dual Outlet