Some additional progress...
I now have my 4.2ghz OC back. However the EFi works very differently in this regard. So far I have not been able to get an SSDT working (all the SSDT i have access to were made pre EFI, so i suspect that is the main problem). Therefore i have only 2 p-states, the highest (42x) and lowest (16x) and the system toggles between the two as needed. This is the same behavior that was common when using DropSSDT flag and no SSDT.aml with the pre-EFI bios, so nothing is really different in this regard. Using an old SSDT results in a kernel panic at boot with a p-state error.
In addition, if i try to OC by boosting the turbo frequencies, which was the method employed pre-EFI, i get stuck at 34x max in OS X no matter what i set the turbo ratios to. Therefor at least in this current SSDT free state we are in the only way to OC is via increasing the default multiplier and leaving all the turbo states at AUTO...
As many know, the old behavior with gigabyte boards was that increasing the default CPU multiplier would result in a KP at boot. In fact many people had to reduce the default multiplier below the actual CPU rated one to avoid this problem. All overclocking had to be done with the turbo ratios because of this issue, and that method worked well since it would only turbo up when the CPU was loaded and the higher speed was warranted.
The EFI does not have this limitation. You can adjust the default multiplier up, and there are no KP at boot from doing so. This is the only way i have found to overclock the system for now. And for all practical purposes the behavior seems just the same, where it toggles between high and low p-states depending on load. So CPU is not running any hotter than before. I do wonder if/when we get SSDT working for additional p-states if it will work as well with a modified default multiplier as it did with the modified turbo ratios. Hopefully it will, and that will be that - or maybe with a proper SSDT it will allow turbo ratio overclocking to work again. In any case thise seems to be the best option if you want to keep an OC and gain the other benefits of EFI.
One last note. using this method, 'about this mac' no longer displays my OC speed, rather it shows the normal default speed for the CPU (in my case 3.41Ghz, a speed in fact my system never hits due to the 42x 16x toggle limitation). Maybe that's something that can be fixed in chimera.
g\