Yosh did good things for the 09/10 CRF450 and I'm hoping for the same results on my Yamaha. The slip on is actually slightly heavier than stock (less than 2 oz.). The RS4 has a very clean look and it's more than 3 inches shorter than stock. I don't like long mufflers because the more that sticks out the more likely you are to bend stuff when you crash. That's another thing I didn't like about pre 2009 YZ450s. Anyway, I'm expecting more top and overrev.
That looks sweet. Have another pic from the inside? I'm still waiting for their full system as I'm in no rush with winter here and not being able to ride untill late april anyway but their slipon should perform well with its resi chamber before the main silencer.