I've been using Speedplay X-series pedals for 5 years now. I love them. No knee problems. I'd previously used Look pedals with 0 float (the very early clipless) and had knee problems. I haven't found the unlimited float of the Speedplay's to be a detriment or problem in any way. I haven't tried to gage efficiency, but that may depend on your riding style and how often your feet twist involuntarily and how much you need them to twist to avoid knee pain. Is this an issue purely an efficiency issue (ie. an optional correction for you) or is limiting the amount of float truly better for your knees? You might try training a video camera on your feet while you ride on a trainer with the Frogs just to see how much your feet are really twisting through the stroke in different situations (spinning, mashing, standing). Maybe limiting float to 6-8 degrees wouldn't change your pedaling at all.