There's an interesting article in The Wall Street Journal about vegan and vegetarian diets for athletes. The article doesn't suggest anything conclusive, though:
"There's no evidence a vegan diet can improve an athlete's performance, says David Nieman, a professor of health and exercise at Appalachian State University. His 1988 study of vegetarian runners found they ran as well as their meat-eating rivals but no better. Although the vegetarian athletes in his study also ate eggs and dairy foods, he says, 'there is scientific evidence that veganism, when done right, won't hurt performance.' But, he adds, there is only anecdotal evidence that it can help."
The article focuses primarily on the health impact, though notes that mixed martial arts fighter (or should it be mixed martial artist?) Mac Danzig became a vegan for animal rights reasons.