I guess it all depends on what you like and what kind of riding you do. I have a shock bracket on mine that allows me to run different length shocks and I have a set of 400EX shocks on it now but I plan on looking into a set of 450 shocks with rezies. Even the 400ex shocks were an upgrade having more travel, I am sure the 450 shocks will be even better.
450R YFZ's will be a huge improvement, one of the best stock shocks out their outside of big buck shocks. Do what ever you have to do to make them work. I moved my lower mounting points 3/4" to get them right.