Something I would like to add that I don't think has been touched on before:
linky. This is one approach to fitting a magwell to a pistol, and can be used on Hi-Capa's as well.
Speaking of which, I am preparing to take the plunge and properly fit a magwell to my current Hi-Capa. I'm thinking that so long as I go slow - i.e. remove plastic grip material slowly, and check the fit lots - I should be just fine. Any other tips for doing this with a Dremel?