Can you link me someplace that talks about how this bios option affects OSX? I wasn't aware there was an impact other than allowing USB devices to function before OSX boot. I've not noticed any speed impact, but I don't use USB drives too often. I do have a 16GB flash drive that works at 33MB/s (peak sequential speed, tested with xbench) and an EyeTV device, but those are the only things I ever use that would be impacted by speed. The rest is keyboard, mouse, IR, a scroll wheel, and other random stuff.
Just to be clear, I don't doubt you. It's just that this is news to me, and I'd like to learn more.
Oh, just happened to think. You're not confusing this with Hi-Speed/Full-Speed setting are you?