FWIW, I wore Shimano mountain bike shoes for the first year or so after I got clipless pedals for my road bike and I don't think I had problems with stiffness. I rode two centuries in those shoes, plus lots of training miles. This was back in 2003 so my memory about the shoes is hazy. I think I had some pain with my left foot after the first century, but I attributed that to the way I rode at the time -- with those cleats I clipped out with my right foot at the top of the pedal stroke, so anytime I was nervous about needing to clip out quickly I would coast with my left foot at the bottom of the stroke, and because I was very nervous during that ride I tended to press very hard with my left foot.