I recently wanted to try SPD-SL system. I really like the wide pedal platform and the ease of clipping in compared to my Look Keo Classic. But the shimano cleats with supposedly 6 degrees of float suck.
I used to use the Look Keo gray cleats with only 4.5 degree float and the shimano yellow cleats (with allegedly 6 degrees of float) feel a lot more restrictive.
Do these loosen up after several hundred miles, or should I sell the pedals (1 Ultegra 6700 and 1 105) and move onto Look Keo Max platform? Or try Speedplay at last.

Perhaps the alignment of the cleats on the bottom of the shoe is such that your natural foot angle falls in a different spot with respect to the range of float than it did with your previous pedals?

+1. That's a good suggestion. At first, my SPD-SL cleats had all the float on one side rather than a little each direction. After addressing that, I don't remember my cleats loosening much in terms of float...maybe just a little.

Good question, I spent 2 hours aligning the cleats 2 days ago. I think my main problem is that there is not much "free" float per se, it is just sprung float. There seems to be also noticeable friction on the center metal plate.