I did two double centuries with Mavic platform pedals (Look / SPD-SL clones) and cheap Performance Bike shoes and, while I made it to the finish, I was too much out of it by the end in both cases to discern the effect of pedals and shoes.
After that, I did a double and a 150 with eggbeater 3's (cleats near identical and possibly compatible with SPD) and Mavic Tempo MTB shoes. This combination was more walkable and had far fewer failed clip-ins. However, my feet got pretty tired, especially around the contact area, which led me to suspect either the small contact area of the eggbeater/SPD cleat or insufficient stiffness of my shoes. (But then, my shoes were almost as high end as it gets in MTB land.)
I'm wondering if I should give platform pedals another shot, possibly with new road shoes, next time I try a 150+ mile ride.