Find me a professional athlete on a grain-free diet.
Maybe they're out there somewhere, but I'm pretty sure if you find me one I'll find you a better professional athlete who eats grains.
With so much effort put into finding optimal diets for athletes nowadays, if cutting out grains would improve fitness in any meaningful way then we'd know about it by now. If Lance Armstrong or Usain Bolt could go faster with a grain-free diet they'd be doing it.
edit: I found a link about Usain Bolt's diet. Turns out he eats quite a lot of rice:
http://dailyrunningtips.com/training/usain-bolt-diet-food/
Now I can't prove he wouldn't be even faster if he cut out the rice, but I am pretty sure it can't be doing him too much harm.