Wow you used clipless pedals for over five years and you were insecure about getting out of them. How about a picture or a description of what clipless pedals you were using.
I'm pretty sure studies have shown little or no performance advantage to clipless, which agrees with my experience. Still, I like them for the secure attachment in some situations. Like fast road rides, or any fixed gear riding.
Studies are like surveys, they are very selective in what they study. Pedal force in the down stroke is the same clipped in or not. There is more to it than this. On flat pedals you can't hold the proper foot position on the pedal constantly. This becomes even harder when you use more muscles by pulling and pushing through most of the pedal stroke. Also when you are on a rough road. The bicycle shoes are also designed to allow you to ride and not fatigue your feet. So if you don't use clipless pedals a good pare of properly fitted bicycle shoes can help. I like clipless pedals for all the riding I do.