If the 5700 XT was working for you and sleep hibernation worked fine with it. Do you change anything since then ? can you back track any changes or steps u took when u got ur 5700 XT ? Maybe with the 6600 XT you have to undo changes you may or may not have made since getting your 5700 XT ? Please keep us informed !
Here are all my changes since I swapped 5700 for 6600 XT:
AGPMInjector's injected device id change from 5700 -> 6600XT
dGPU DeviceProperties in config.plist from 5700 -> 6600XT
Resizable bar in BIOS, Above 4G decoding from disabled -> enabled, ResizeAppleGPUBars set to 0
I've tried several times with different settings but still no luck:
Disabled resizable bar, above 4g decoding, leave ResizeAppleGPUBars to -1 (default value)
Disable AGPMInjector
Disable WEG and boot without arg agdpmod=pikera
Boot without any dGPU DeviceProperties such as cosmetic properties and amd radeon framebuffer inject (use Henbury instead)
Boot with other args for black screen patches by WEG agdpmod=ignore
Disable HibernationFixup
Boot with other args for HibernationFixup patches -hbfx-disable-patch-pci
Change HibernationMode from NVRAM to Auto/RTC in config.plist
Most symptoms are like system hang (needs to press reset button, no network activity) without screen wake after boot from hibernate state
Few times result in auto-restart to boot with apple error report message saying that windowserver timeout for 120 seconds