@Traumflug If you look at the tests there is virtually no difference in using 6 files as there is 1 file. This is what leads me to believe in future versions we need to make multiple css files to improve paint speed. Just splitting the addcss hook into a top and bottom version should be enough, there is no reason for footer modules to load css at the top of the page. I think a lot can be deferred like this is we add the options and module developers are more conscious of it.
As for removing unused selectors, that is something I have looked into heavily. I don't think there is any way to do this. There are a few unmaintained projects that have tried this, but have failings. At the same time you can do it with chrome programatically too. But the one thing they all lack imo is handling css in relation to javascript. The product page is a great example. You can strip the css down on that page using one of the tools, but then when you add the append for the thickbox, that css has been removed and it is unformatted.