The horse is supposed to chew the bit softly, and they do so by thrusting the tongue (a snaffle does rest on the tongue). Sometimes early on they roll it around/chomp before settling in. (And it is the handlers job to work the horse in hand to teach it HOW to respond to the bit properly as well.) If it is hung too low or is too narrow, it can be raised up too high (to teeth). Why not just 'try it out' with someones plain old (fulmer) snaffle instead?
Very few horses have a problem (unless it is a sausage tongue with a low palette)...more have a problem with bit used on tongue. And a horse which has had a straight bit will have different reactions. The question is how the bit is used (on the corner of the lips) or onto the bars.