I successfully managed to enable above 4G decoding on my Asus P8H67M Evo using the following procedure:

- First get a copy of MFR's ROM image (version 3703), Dump the setup IFR, and find the varOffset for the (hidden) "Above 4G Decoding" and CSM Enable/Disable menu options.
- Boot into EFI shell, and run RU.EFI
- Edit Setup UEFI Variable:
o Set offset 0x253 -> 0x0 (disable CSM)
o Set offset 0x2 -> 0x1 (enable Above 4G Decoding)
- Ctrl+W to save and then reboot

This actually worked - my two GPUs now have 8GB PCIe BARs located above 4G! Yay.

But there is one entertaining side effect:
I no longer see the BIOS bootup screen and when I (blindly) hit DEL to try to enter setup, I never see any video output.

I had specified the integrated graphics to be used for bios output... but seems it no longer works? But everything is fine as soon as the OS starts booting. I wonder if the UEFI/BIOS needs video card located below 4G or something? Any suggestions how to correct this (another hidden setting?) would be appreciated.

it doesn't work with CSM?

@bgelb0 The 3703 BIOS does not contain any iGPU GOP module so the onboard graphics cannot output an image with CSM disabled, this is to be expected.

Intel 6-Series chipsets were the first to implement UEFI and many are only Class 2 (UEFI with CSM) as they are missing some UEFI modules (such as GOP and SataDriver etc.).

You can work around it by using a discrete graphics card that does support Graphics Output Protocol (as you have probably noticed).

Thanks for the explanation. I think this lines up with what I am seeing. After some additional fiddling, was able to get video out from BIOS on one of PCIe-attached graphics cards.