What kind of exercise saddle is best for race horses?

Does anyone have any thoughts on what is the best type of exercise saddle to use on a thoroughbred? Full tree or half tree? I've ridden jumpers for years, and I know saddle fit can really affect the balance of the rider and the comfort (and movement) of the horse, but no one seems to consider it in the racing world. While thoroughbreds (especially in North America) tend only to be under saddle for 30 - 45 minutes, I have no doubt that a poor fitting saddle can really inhibit their movement. I'd think that a full tree saddle would be better for distributing the rider's weight than a half tree, but full tree saddles only seem to come in one tree size (narrow) and thoroughbreds are not all built the same. Half tree saddles (which is what I've always used on the racehorses) fit a greater variety of horses, but would concentrate the rider's weight over a smaller area. It would be interesting to hear some of your thoughts on the matter. Does anyone have any experience with sore backs in race horses?