The general rule of thumb is 20%. This guideline was developed by the cavalry for fairly hard use (traveling long distances over rough terrain) and doesn't take into account the horse's conformation. Certain conformational features- such as a long back, thin legs, and a high center of gravity will decrease how much a horse can carry, while the opposite features (short back, thick legs, low center of gravity) will increase it. Typically, a 15.2hh 1200 lb QH will be able to comfortably carry more weight than a willowy 17hh TB that weighs the same.
QH's are very popular for barrel racing, and it sounds like weight-wise you'd be fine with even a smaller 1000 lb one. I'd pay special attention to leg/joint conformation, since barrel racing can put a lot of strain on them (regardless of the weight they're carrying)