Re: Stripped down Per-feature version of PIE
I don't know an exact number, you'd have to try it and see what the savings would be. The only feature that's totally standalone is border-image; all others depend to some degree on at least border-radius because it affects them (box-shadow matches rounded corners, etc.)
I'd suggest that you check out the GitHub source and start stripping out the stuff that you don't want. I'm interested to hear your results of file size savings.