I can never understand why western riders insist on using curbs/shanked bits for barrel racing. Most direct rein around the barrel. A curb bit/shanked bit isn't made for direct reining - it's made for neck reining, and very subtle neck reining at that. It's very hard to actually pick up a workable contact and influence your horses shoulder straight awaya nd clearly with a shanked bit at any kind of speed.
I don't barrel race regularly, but when I do I do sucessfully, and always in a snaffle. They are made for that kind of rein contact and send perfectly clear signals.
I personally wouldn't barrel race in a hack if I were doing it competitively, which it sounds like you do. They just don't have the refinement of a bit, expecially in relation to the shoulder, which can make or break your run in a milisecond.