To me the efficiency of clipless is not in power output but in not needing to think about foot position.

If you set the cleat to the best position your foot will go there every time, all day. That eliminates the need to check or adjust foot position which over many hours can add up.
If I get a cramp or hot foot I can lift my foot while pedaling and wiggle/flex it.

On the flip side, If you tend to stop and start a lot and/or get on and off the bike regularly, clipless may not be more efficient as you have to think about clipping in and out.

With my long distance road tour bike I am clipless. On my off road bike and my commuter I am platform. Plus platform allows me to wear trail running/hiking/work shoes.