If this is why you're moving to SPDs, then I think you'll be sorely disappointed. Both my own personal experience (some of my bikes have SPDs, some toe clips, some flats) and various published studies show that there is little to no measurable efficiency gain to be had from clipless pedals and somewhat surprisingly, pulling up on the backstroke (whether with clips/straps or with clipless) actually decreases power output. The main gain from SPDs is ease of entry and reduced foot pain due to tight straps (which I don't care about since I no longer tighten my straps).