I'm not sure the exposure is better on the K20D? Shooting Raw would make it very easy to get what you wanted on either one of these...however, if I am not mistaken, f 5.6 and F7.1 do not deliver the same amount of light anyhow?
What I have noticed...personally....it is harder for me to get what I want from my K5 when shooting people than it was with my K20D. Since I have been able to get some from my K5 that met my expectations, I feel sure this is due to some differences in the two cameras that I have not adjusted to....after all, I took thousands of shots with the K20D and learned its behavior...have not done that yet with the K5. It was the same when I went from the K10D to the K20D....I conclude that this is normal procedure with a new camera? Or, it could just be the damn Squirrels have ruined my ability to shoot people......?
Best Regards...take some more and post them, we will tell you what we think and someone with more knowledge than me will help you adapt to the K5, it is a fantastic camera...in case I haven't mentioned that.
Best Regards!