Virtually zero, according to the research I've seen. Foot retention was not, as far as I know, ever designed to increase efficiency. Your chain rings, you see, are round. You don't have to encourage them to go in a circle, and pulling up is very inefficient except for *maybe* that split second as you pass through the 'dead spot'.

All of these retention systems are designed simply to stop your feet from coming off the pedals under hard effort, because that can be quite a dangerous thing to have happen!