It may POSSIBLY make your gouping better, but the biggest impact on that is you, the shooter. Proper and consistent form are the recipe for good groups, but proper equipment plays into that as well.

What I mean is, if you cut your current arrows down, you won't be so overspined that it will work against you. An arrow that is too stiff is just as bad as an arrow that is too weak. There's a happy medium there, you gotta be in the right place. With your set up, you SHOULD be shooting 500's, ROUGHLY 22 1/2" long. I can't be sure of the length, it depends on several things that can't be determined here. By cutting your 400's down, you will stiffen them up, but they won't be too stiff. They should work fine.