5xxx maxes out at a worksize of 256.
My dual 5870 (w/o CF bridges) maxes out at WORKSIZE=128.
Nope, that too maxes out at 256. What I said was 768 simply is not valid for 5xxx hardware.
Phoenix should output the error OpenCL is returning instead of covering it up.
I'll probably add this in the next version, but for now it just uses the maximum supported if you enter a higher value.