I thought I'd add another note of thanks to
@deeveedee. Your guides, comments, and EFIs were an incredibly helpful reference when I first set up my hack last November and I've been following your changelogs with interest ever since.
I'm using an HP Z1 Entry Tower G5, which is basically a rebadged HP EliteDesk 800 G5 Tower -- the Mini's bigger and uglier cousin -- but nevertheless many of the things discussed in this thread have been relevant to my system. My hack is also a bit unusual in that I use an eGPU with Vega 56 in macOS -- I recall there being some comments about eGPUs a few pages back.
The hack has been my daily driver since January and it hasn't skipped a beat from 11.0.1 -> 11.3.1 (and OpenCore 0.6.3 -> 0.6.9). Not even a single kernel panic.
Take a quick look here:
https://www.cpu-monkey.com/en/cpu_benchmark-cinebench_r23_multi_core-16
You can see that the 9700 expected cinebench score should be in the 8000s. You are not far off. I can see during my test that the CPU is throttling due to temperature (I monitor the temperature at all time) and the 9900 is supposed to hit 12000+, It should be >50% higher than the 9700 but I am only at 9000. It seems like the 9900 is too much heat for these little boxes to handle.
The i9-9900 in my HP Z1 G5 (with tower case, stock HP cooler rated at 95W, and much more thermal headroom than the Mini) scores 11188 in Cinebench R23, which is still a bit lower than those predicted scores. I don't see any thermal throttling -- temps don't go much beyond 80°C -- but rather the CPU seems to be power limited by HP in the BIOS to around 116W even when PL2 is set higher.
Another option is to limit TDP with Intel XTU a few watts lower to prevent thermal throttling.
On my system, my settings in Intel XTU (on Windows) don't carry through to macOS. In macOS I use
VoltageShift.kext instead to set PL1 and PL2. Without XTU or VoltageShift, PL1 for my i9-9900 is at the default 65W which leaves a lot of performance on the table!
Experimenting with that PL1 value might be worth exploring if anyone wants to eek out some extra performance from their Mini.