I cannot understate how Slicy has made my life a million times easier. Even without it's retina stuff, it's a huge timesaver.
Great to see your workflow! Your approach to doing backgrounds in Sass is a new one to me; I've been using some other mixin that does ok, but it is finnicky. I will definitely try your approach out.
This is excellent. Have you worked with SVG at all?
I'm moving away from PNGs on the web and towards SVGs, which doubles the amount of work but produces better results when scaled.
PNGs, for some reason, get occasionally mangled by webkit on non-retina screens when scaled 50%. It sucks.
I have but was experiencing the same thing, really crappy subpixel rendering from certain browsers. Having two separate, pixel perfect images still works best in my opinion.
Simurai wrote an interesting post about this: http://simurai.com/post/19895985870/icon-sharpness-limbo