Method 2 is definitely working for 58xx cards
That said, Method 2 does not appear to work with the Sapphire Vapor-X 5750. If I recall my (very late night) testing yesterday, Overdrive would still load and appear to be activated, but would not apply new settings, nor would it show the new limits. Reinstalled drivers several times. Reflash to original BIOS resolved the issue.
Oddly enough, GPU-Z seemed to report the card running at whatever I set the max to. By which I mean, using no modifications other than Method 2 bumping the Overdrive limit to 1Ghz, the card would load up and run 1Ghz at idle, despite the actual Overdrive panel showing stock settings and stock limits.
(Tried Method 1 as well, but having read the thread, I realize now that it shouldn't!)
Voltage control does not appear to work either. Any modification will cause the card not to recognize and drivers will not activate. I know the standard Sapphire 5750 uses the uP6201BQ controller, which is officially not able to be programed, though I'll have to check and see what it is on the Vapor-X when I'm home...
Would be happy to post up my original and/or modified BIOS if you'd like to look into it.
[EDIT] Damn! uP6201BQ on the Sapphire too... Call me ignorant, but if these things don't have software control, how is it that they can adjust the voltage down for PowerPlay? [/EDIT]