Running barefoot is associated with a substantially lower prevalence of acute injuries of the ankle and chronic injuries of the lower leg in developing countries, but well-designed studies of the effects of barefoot and shod running on injury are lacking. Laboratory studies show that the energy cost of running is reduced by about 4% when the feet are not shod -sportsci.org
MY COMMENT: This doesn't necessarily mean you can decrease your race time by 4%. However, last month I struggled to run 18:26 on the roads. By comparison, last week my 17:47 barefoot run on artifical grass was easy. That's a 3.6% improvement.
Now, if only more races were run on smooth, forgiving surfaces.