If any recent purchasers are having performance issues with the Mac 285, one possible source of problems is the throttling back of the memory and shader clocks when the card is not in full 3D mode. The shader throttles back to 0.6GHz and the memory by a lot more. If you try MonteCarlomultiGPU just before and just after running a 3D app (like the benchmark in OpenGL Extensions manager) the result can easily be different by a factor of 10. (10k options per sec before, 100k just after, falling back to 10k via 30k after several seconds).
While we wait for an official fix, I found found that running another small Cuda app in a second Xterm window that pokes the 3D graphics does a very good job of keeping the card in fast mode. Others may have views about the best app for this, but the rotating blurry teapot (aka postProcessGL) does a good job on my 08 Mac Pro. Most of the time with this running I get about 100k options per sec, though sometimes it falls back to 70k. Screen shots tell the story without and with the teapot - see the clock speed at the bottom of the truncated deviceQuery output and the options per second figure.
Note that other people have raised other perf issues, and do not seem to have my problem on an 09 Pro - see the thread on comparison with GT120 for something completely different.
Suggestions as to better apps (less demanding - the teapot does consume resources - but enough to keep at 1.48GHz) very welcome, as would be an official control panel! Don’t forget to turn the teapot off when you are done, else you might lower card lifetime.