Surprisingly I just discovered that most of the performance loss on my aging MBP was due to the way display clamshell mode works in Mountain Lion. By playing with no external display or by disabling the MBP display with a magnet I got a good peformance improvement. I play with settings on low/fair and it runs at 40-45 fps on average on new Mist Of Panaria zones. Not something to report back home, but it's playable now.