I think that everyone is overly occupied with the up/down impact load ability of the material used in the linkage, hence the preoccupation with the hardness of the material. I think this is wrong. The linkage on the KDX was designed to change this up/down impact energy into a smoother, more progressive loading energy absorbed by 5 different points on the shock, linkage and swingarm. It is not just the two bolts at the top and bottom of the shock doing all the work. One must look at the entire rear suspension system and analyse how the system shares the loading induced by the big mac burgers floating around our guts. Kawasaki use roller bearings out of convenience more than anything else.
Just to set your mind at ease. Check out the site www dot vesconite dot com. Go to top right hand corner, click on "no grease suspension" then click on "Applications/success stories" then click on the articles on suspension applications. You will be sold on it immediately. The material is used by Toyota, Colt, Isuzu, Nissan, Mazda, Ford and Pajero on the suspension shackle bushes on their rally/offroad vehicles.
Read the article on "suspension bushes for heavy transport vehicles", If it works on a fuel tanker with a few tons of load as well as Bell Earth moving equipment, then it will do just fine on my 100kg KDX with my 85KG backside. As I said before "throw the roller bearings AWAY"
Ultimately the material you chose to replace the roller bearings is up to the individual. On previous boneys, I have used brass and bronze bushes, which have worked just as well and since they are metals, people tend to trust them more than plastic.
Whichever material you chose, the aim of the exercise is to remove the unreliable, grease hungry roller bearings and replace them with reliable, low maintenance, cheap, one piece bushes.
Vesconite in the US - For more details contact Tony Ball, at 561 736 3950 or e mail tonyball605 at bellsouth dot net