First of all - thanks for the useful plugin!
I have a minor observation:
I noticed that stylesheets are being grouped by "media" attribute, which is the right way to do it. I also noticed that there are quite a few styles that don't have specified "media" attribute (usually inserted by other plugins) and, by default, their media parameter is set to " ". In your case they are being minified and printed separately as media="0". I think it is save to assume that their media attribute could be marked "all" and combine them with the rest of the "all" group.