The Motherboard is customized for the case, so an aftermarket board would not fit due to the IO shield not matching up.

DELL does not sell motherboard, so you are limited to the board you currently have.

The Alienware machines use OEM processors, not retail. That might cause you to run into issues with processor support.

Furthermore, based on some testing by other users, the R12 might not support 10 processors, but only 11 series. Page 12 this manual: R12 specifications 

The R12 has some performance issue due to the gear mode, as reported by some owners of R12 machines. This seems to be mostly related to memory latency issues, with higher than expected memory latencies.

Your underperformance might be related to that specific issue.

For AMD you would basically have to switch to a different motherboard, and that will not be possible in that case. If you want to switch to AMD you either would have to order an R10, or build your own and recuperate the video card and other components from your R12.

For memory, only the following memory is validated and guaranteed to work. Any other memory might, or might not work. These systems are known to be picky when it comes to memory support.

Validated OEM memory =
Four slots
128GB maximum
6TCK6 32GB HyperX Fury XMP DDR4, 3400MHz, Unbuffered, 288 pin, Dual Rank, 1.25V, Non-ECC, Kingston X6TCK6-MIE, X6TCK6-HYA
VTW4H 32GB HyperX Fury XMP DDR4, 3200MHz, Unbuffered, 288 pin, Dual Rank, 1.25V, Non-ECC, Kingston XVTW4H-MIE
JV223 16GB HyperX Fury XMP DDR4, 3400MHz, Unbuffered, 288 pin, Single Rank, 1.35V, Non-ECC, Kingston XJV223-HYA, XJV223-MIE
K2M26 16GB HyperX Fury XMP DDR4, 3400MHz, Unbuffered, 288 pin, Single Rank, 1.35V, Non-ECC, Kingston XK2M26-MIE
2YH1K 16GB HyperX Fury XMP DDR4, 3200MHz, Unbuffered, 288 pin, Single Rank, 1.25V, Non-ECC, Kingston X2YH1K-HYA, X2YH1K-MIE
W21KG 8GB HyperX Fury XMP DDR4, 3200MHz, Unbuffered, 288 pin, Single Rank, 1.25V, Non-ECC, Kingston XW21KG-HYD2, XW21KG-MIE, XW21KG-MIE2

KF processors aren't OEM, you can pick up a 11700KF at Best Buy if you want ( I did too replace the 11600KF that came in my R12 )

That would be a retail 11700KF, also sometimes referred as "boxed version"

OEM is a 11700KF that is sold as "tray version".

There's technically speaking no difference in the actual processor, but the ID string might be different between a boxed version and a tray version.

Intel link that explains it a bit better: OEM versus retail Intel CPU's 

They'll work, only issues R12s have had is with downgrading to 10th SKUs

G'day,

This dialog has is very instructive and I think I know the answer to this question, but I have to ask the experts.

My R12 currently runs i9-11900KF @3.5MHz, which has been very satisfying.  When I look at the CPU benchmarks for newer CPUs such as i9-12900KF, they are dramatically better: 40853 vs 25656.  

The i9-11900KF is chipset Intel Z490, and I believe the i0-12900KF is z690.   

Any chance of a CPU swapout?  No....regretably thought not.

Thanks for your thoughts.  

Not supported.