I actually had Speedplays with 15 degree float and they attributed to my knee problem. I overpronate and have a tendency to ride with my toes pointing away from the bike rather than straight forward as they should be...
Upon getting a professional fit I was told to go to Speedplay Zero's immediately, and to set them up to NO float... knees have been great ever since!
Everybody is different and the type of clipless you go to CAN alter your fit slightly so be sure to get it double checked too! You may have to raise the saddle in addition when ya get 'em...
There is a fine line between "hobby" and "mental illness".