Only here on CC would a student describe his ACT score of 31 as low!
Okay, I get it. Since the OP is aiming for “top 20 schools” (whatever that means), the 31 would be on the lower end, but the OP needs to realize that if the rest of his application truly is excellent, he might be able to get schools to look past this, especially if liberal arts colleges are in the mix. Schools like Williams, Carleton, Wesleyan, and Bowdoin have strong programs in neuroscience and regularly admit stellar students whose ACT might be lower than the 50% midpoint for the average accepted student. Emory, too, could be considered. I would hope that the OP realizes that there is really no such thing as a “top 20” school. If the OP is pre-med, perhaps he should look at schools where he will most likely succeed at getting into medical school instead of becoming yet another once-hopeful pre-med who gets weeded out.
Finally, yes, ED will give the OP a small boost, and he can even research admissions statistics to find out where his ED chances would be greatest.