BYU enjoys talking about the High GPA of their incomming classes. But everyone understands that GPA-measures vary from school to school, and that it isn't a great indicator. Also, EVERYONE applying has such great GPAs, that it doesn't differentiate much. And a few students with lower GPA's aren't going to 'mess up' the average of the admitted class.
Go get a 30+ on the ACT and don't fret. ACT may not be a perfect indicator, but it is (IMO) clearly the most important factor (assuming ecclesiastical endorsement) for an applicant.