Performance difference between Shimano SPD and SPD SL
There likely isn't much difference in performance. Some people do find that SPD cleats lead to "hot spots" (painful spots) on the balls of the feet; that's why I went to SPD-SLs. 

You might find a difference in platform height; IIRC the height of the SPD cleats + pedals is shorter than the SPD-SL. You'd want to adjust your seat height to deal with that.