@tekwendell xen carefully manages CPU power management to match VM load and vCPU count, I would not manually try to adjust things with xenpm in the meantime as it's likely you'll make things worse (don't try to outsmart xen power management unless you have a VERY specific use case). Xen is designed for paralleled workloads (more than a single VM), so there's many tunables for VMs that are set with this in mind (like CPU affinity). So by default I'm sure the CPU affinity for your single windows VM is still set somewhere in the "middle", so it's not going to be allowed to schedule the full CPU time versus what dom0 is also using.
I'm not an expert in AMD/Epyc power management, but I believe it's pretty typical that CPU power/clock management boosts based on overall CPU load, and running a benchmark on only a single VM using something like 8 cores on a 64 core processor is not going to demand a lot CPU time, so I'm not surprised to see it's not boosting very far. Spin up 6 more of those VMs and benchmark them all at the same time, I wouldn't be surprised if you see it start boosting higher
475 cpu-z versus 501 bare metal is very good and indicates pretty clearly there's no issue here, you're getting 94% bare metal performance on windows under a large virtualization stack (historically the OS with the most overhead to virtualize). I would be very happy about this
If you really want to dig further, ensure your bios power management is set to "OS-controlled", this will hand more control over turbo and c-states to the xen power manager and is what is recommended on AMD processors, and then you can use some commands listed here to check actual turbo status. But again, note that I won't be surprised if you can't get a 64-core processor to enter its highest turbo states when only stressing 1/10th of its cores: https://support.citrix.com/article/CTX200390/power-settings-in-citrix-hypervisor-cstates-turbo-and-cpu-frequency-scaling