That's possibility #1 in my mind. #2 is that you'll want to be absolutely sure there's no overlap anywhere on the shim if you cut it and it 'springs' down smaller than it's original shape.
My final thought is that I suspect a minute gap in the shim resulting from a clean cut won't hurt a thing. However, that's given a clean cut; leave a burr anywhere on the edges of the cut and now you don't have uniform pressure behind the barrel extension shoulder. Something to keep in mind.