There have been several athletes who have been able to perform at a high level while dealing with Chron's disease:

Jacksonville Jaguars QB David Garrard

former Hartford Whalers RW Kevin Dineen

former Calgary Flames RW Theo Fleury (who did struggle with drug & alcohol abuse, but that may have more to do with him being sexually molested by a junior hockey coach)

Ironman triathlete Jonathan Caron

Professional wrestler George "The Animal" Steele (I'd imagine that eating turnbuckles might have resulted in inflammation of one's digestive tract)