I have a 5870 in my Mac Pro1,1. I tried out DiabloMiner.app in 10.6 Snow Leopard, which yielded an average of 140MH/s.
After an upgrade to 10.7 Lion, I now see an average of 273MH/s.
I'm not exactly sure why the change appears so drastic. Perhaps it was an upgrade to the ATI drivers and/or OpenCL framework. Or perhaps 10.7's UI is just much more efficient with its use of the GPU for acceleration. (Java version is still 1.6)
Either way, it was a welcome bump in my mining efforts.
But I think perhaps it could still perform better by booting into Ubuntu.