Originally Answered: Would the average athlete be better off if he/she ate a vegan diet prepared by a nutrition expert?
Edit: Short answer? Yes.
The data out there notes that athletes have noticed quicker recovery times and more alertness when on a well-planned vegan diet. Also, there is plenty of research to suggest that there are large numbers of health risks inherent in a meat and dairy consuming diet that would obviously be avoided in a vegan diet.
Also, the idea that protein only comes from soy and beans has long been debunked. Protein can be obtained through lentils, beans, nuts, seeds, fruits, vegetables, and soy products... all of which have shockingly less pesticide and chemical residues than dairy and meat products.
In fact, if you focus on the information available on MyPyramid.gov (don't go to .org - it redirects you to a website touting the corruption of the USDA..), the revised pyramid and accompanying food information meet up with the recommendations of a vegan diet anyway. The dairy section now focuses more on "calcium-rich" foods, and the protein section more on variations of choices... including nuts, seeds, beans, etc.
There are some diet-specific problems that athletes may encounter when beginning a vegan diet: low dietary protein, low dietary fat, low sodium, low calcium, and anemia. These are problems that are all avoided with proper planning (by a nutritionist) and including the correct grains, veggies, fruits, etc in the diet.
Here are some specific tips for vegan athletes, so they can get started off right: http://vegkitchen.com/tips/vegan-athlete.htm