Here is a little trick that works for me on SL 10.6.8:
During my investigations, I found out that most of the graphical problems in SL seems to be caused by the web browser engine.
So the trick for using MC Mac with SL is to stay away from views that are using the web engine, mainly the start view.
Go to Tools -> Options -> Startup -> Location and set startup location to 'Audio' instead of Start.
Close and relaunch MC MAC.
This way when you start MC Mac you'll be in the Audio sections where most views Album, Artist, Panes, are working Ok.
It should allow you to explore most of MC Mac features under SL while the good folks at JRiver are working on a better solution for SL.
EDIT: and I did not notice any sound quality differences between 10.6, 10.7 or 10.8 that I am using on different macs. But my ears might be on the oldish side!
have a good day,
Christian