I don't think you're being limit by 8x at all. I'll bet once you get 16x going, if you bench both ways, you'll see no difference.

As for the locked out setting, I was on F5 and just updated to F6 to check, but my options for changing Turbo/SATA3 are unlocked and selectable. I tried plugging things into the GSata and USB3.0 ports, changing all kinds of config options trying to see if something would make it unselectable, but I can't duplicate your problem.

Have you tried resetting CMOS and see if that opens it up?