I use a set of duratrax carbon shoes on my K3 with mugen 1.0 springs. It works great and is readily available and cheap enough. The duratrax shoes you have to dremel the slot in the middle so it's not off to one side. And the mugen springs you have to dremel like 1/16 off the length of the straight leg. Takes about ten minutes to do all the work.
It hooked up great for me. I was using all aluminum shoes but got tired of the maintenance that goes with them. You need to take em off and clean the flashing after every day. And the middle groove needs filed on the edges so the flashing doesn't bind up the spring.
I didn't liket he 1.1 springs, but that was with my aluminum shoes which were near worn out. Had to give way to much throttle before they would engage. They would probably be ok with new shoes that had more mass to them as they'd open up sooner.