Tested HWP on my Skylake NUC6i7KYK.
I get better GeekBench4 with just XCPM instead of HWP.
iMac17,1 with HWP enabled: 4235/14055
iMac17,1 without HWP (XCPM only): 4614/14652.
iMac14,2 (XCPM only): 4598/14686
By "XCPM only" I mean that I'm just injecting "plugin-type" via SSDT-PluginType1.aml (from my github repo)... eg. not using SSDT.aml from ssdtPRgen.sh (which [guessing] seems to be not needed on Haswell and later...)
I verified that HWP enabled SMBIOS do not have APSS/ACST/etc in their ACPI set. And even though iMac17,1 is not HWP enabled by default, its ACPI set also does not include APSS/ACST... Older XCPM enabled machines do have APSS/ACST, but I suspect it is not actually used...