Enlarging the prop cutout can either take it further out of balance or help balance it. As you know you don't want the prop to touch the spinner in the cutout. How much clearance that you have is a little more ambigous. If the spinner isn't far out a little off all around the cutout may fix it.
If you drill holes in the back plate to balance it be sure that the cone attaches to the backing plate the same direction each time.