I'll just share that I use SPD cleats so that I can walk, and even drive if necessary. I'm probably one of the few that do, I'm not sure how much performance one would gain from the SPD-SL system. Many SPD-SL shoes have companion models with SPD cleats, so you can still get lightweight, stiff shoes designed for road use. Shimano also makes the single-sided PD-A600 SPD pedal specifically for road use, so it doesn't have the extra weight associated with MTBs. I ride this setup on my CAAD12/Utlegra and it works great.