Others with more experience can add their opinions, but I wouldn't attend an HA showcase with less than 1,250 SAT or 27 ACT. Of course those numbers won't impress the most selective schools, but I think there will be some schools that can at least start a dialogue at that level. GPA is much more subjective, but if you're on the borderline with standardized testing you should probably be better than borderline with grades and AP classes.
A D3 HA coach told my son and me that GPA is the most important qualifier for him, because by at or near the end of Jr year (when D3 HAs get serious about recruiting), GPA is essentially set and can't change significantly; but a student can always re-take the ACT or SAT. (This was at a school with very high grade and test score averages, and one that does not bend those much at all for athletes, so take that into consideration.)