Macbook Pro 2011 with ATI 6750M

OpenCL GPU is 2.7X faster than Flam3.
OpenCL GPU is 3.6x faster than OpenCL CPU.

Macbook Pro 2009 with Nvidia 9600M

OpenCL GPU is 3.1X faster than Flam3.
OpenCL GPU is 4.4x faster than OpenCL CPU.

Macbook Air 2011 with no OpenCL usable GPU

Flam3 is 1.4x faster than OpenCL CPU.

Conclusions

Note that the Macbook Pro 2011 OpenCL GPU is 7.3X faster than Flam3 on a Macbook Air 2011. So, a better Mac makes a big difference in performance.

Flam3 is consistently faster than the OpenCL CPU renderer. The OpenCL density estimation algorithm is culprit here. But Flam3 is consistently slower than OpenCL GPU.