Well, there is also nox, but you pretty much covered it there. Darn, I can't think of anything else to do. If you are really crazy gung-ho about Sabayon you can install gentoo and then hop to Sabayon from there, but it will take effort.
I don't have the same AMD ATI GPU but, just in case it helps, xdriver=radeonHD did not work in my case but xdriver=radeon did work for SL 4.1 (I didn't need to do anything for SL 5.0). So give xdriver=radeon a try too if you haven't already. But, as crisandbea wrote, xdriver=vesa is a good one to try, as it is a 'lowest common denominator'.