Fixed my problem with the benchmark. Not sure what happened, but I uninstalled AE, re-installed it fresh, applied the three updates to 11.03, and added my GTX 580 to the list of GPUs. Downloaded a fresh copy of the benchmark, and now it works again. In particular, works very well. Now I get a time of
5:43 using a MVC-flashed GTX 580.
To achieve this improvement, I did the following: re-booted my Mac (see specs below) and then systematically quit every single add-on I could find - all my servers, all the menu bar doo-dads, my anti-virus, everything. That's when I opened AE and adjusted the Texture Memory RAM available to the GPU to the max, 1535 MB (see below). Finally I opened my free memory app and repeatedly freed up memory until I had over 22GB free, and closed the app. I was ready.
The benchmark ran faster than I'd ever seen before, getting under 6 minutes for the first time. I'm stoked to see what it'll do in a couple of days, when I add a GTX 570 card to the system. Not as fast as Gymnut, but then again he's running a dual hex 3.33 machine with 32 GB, and I'm running a single hex 3.33 with (just!) 24GB. Either way, I'm satisfied.