Did you install the cleats yourself? If you install them too tight, friction will stop the spring from performing properly. I believe the proper torque is 20 inch pounds. I run speedplay zeros on two bikes with one pair being really beat up and I've never experienced the problems you're having.
Float is not the exclusive source of knee pain. Saddle height and fore/aft position could also be the cause. Maybe consider a fitting before you make anymore modifications.