Thanks for your feedback. Here is some information I've gathered regarding how Windows power options relate to the issue I'm experiencing.
Thus far, changes I've made to Windows power option settings do little more than redistribute the workload among CPU cores. They don't resolve the fundamental issue. Since the EFI security update, 1/8 of processing power is occupied by the kernel. There is no user space process visible in Task Manager to account for this workload.
When my Macbook Pro is connected to power, I observe the following behavior for standard Windows power option settings...
High Performance -> 1 thread = ~100%, Balanced Performance -> 2 threads = ~50%, Power Saver -> 4-5 threads at 15-25%.
Note I say "threads" since I believe the i7 processor has only 4 cores, but Windows task manager shows 8 plots.
Prior to the EFI security update the system was idle with negligible workload on all CPUs.
I do not see the option "Power Options -> Advanced Settings -> Processor you mention. However, when I drill down into menus under Control Panel ->Power Options->High Performance->Change Plan Settings->Change advanced power settings->Processor power management with "High performance [Active]" I see:
Minimum Processor State -> On battery: 5%, Plugged In 100%
System cooling policy -> On battery: Active, Plugged in: Active
Maximum Processor State: On battery: 100%, Plugged in: 100%
Googling for solutions to my issue, I have found no resolution for Windows. However, I did discover that Linux users appear to work around a similar issue by disabling an ACPI interrupt. Perhaps something similar is possible under Windows?
A link provided with the Apple EFI security update mentions the Rowhammer bug and related security vulnerability. An approach to mitigate the risk of this attack is to increase the frequency of memory refreshes. My suspicion is therefore that the Apple EFI Security update has configured an interrupt to increase the frequency of memory refreshes. While OS X has a driver to service these, other operating systems (e.g. Windows, Linux) do not.
If this were the case, perhaps a future Boot Camp driver update might resolve this. Alternatively, perhaps I'll find another solution.
Has this information been helpful? Did I miss some power setting related information you were looking for?