I do think I had a slight improvement in overall performance going from street shoes with toeclips to SPDs. I haven't tried other varieties. But, at least for hill climbing, it is good to have something secure that doesn't pull out... ever (very rare).

There is a lot of debate about overall speed/power for flats vs foot retention. It likely affects the muscle groups that a person uses somewhat, but a large part of the power is cardiovascular limited. So... foot retention may make less of a difference than people believe. At least in steady state.

Hard acceleration may still benefit from foot retention. I'm often in a little too high gear, but my acceleration truly starts once I hear that "click".