Like BennysLace said, Stacy is jumping her full percheron mare at 3'9" currently, and was getting dressage scares in the 30's and 40's... She always brings home ribbons in both jumping and dressage, I think she mostly does 3 day eventing. Here's the link to her blog.
www.jumping-percheron.blogspot.com
As long as you keep your horse IN SHAPE, meaning conditioning for speed and endurance as well as keeping them limber and lightweight, with proper hoofcare, diet and chiropractor work, a draft can do anything.