My HP 8300 MT is on OC 0.6.4 with Big Sur 11.2.1 using iMac15,1. Is there any advantage to using MacPro6,1 or iMacPro1,1? I am not using USBMap.kext yet and wonder if I need it, hence the reason for not using 15,1.
As far as USBMap.kext, I think you can adopt one of the provided USBPorts.kext to fit
your system.
You may have to modify the Info.plist inside of the Contents folder of the kext to fit your
SMBIOS, and Hackintool can produce a prototype kext for you. Hackintools and USBMap
tools aren't so great at dealing with the vintage internal ports of 3rd generation boards,
so you may find it easier than starting from scratch with USBMap.
The advantage of MacPro6,1 is that it is based on a 3rd generation architecture, although
it used a third generation Xeon instead of the common 3rd gen CPUs of the HP8300 series.
Thus, its power management and other features may be closer to what is required for the
HP mobo. The disadvantage of the MP6,1 is that its GPUs were early AMDs with obsolete
interfaces, and some of the GPU defaults are not great. It seems to work well with the
HD4000 graphics of the i7-3770 or i5-3570K, but not so great with Nvidia or later AMD GPUs.
The advantage of iMacPro1,1 seems to come from an earlier time around Catalina 10.15.4
when HEVC (H.265) graphics were enabled only on IMP1,1 SMBIOS with AMD GPU and
disabled IGPU. The advantage seemed to disappear with recent versions of Whatevergreen
with more shikigva and unfairgva options for enabling DRM, also with other more recent SMBIOS
like MP7,1.
An advantage of IM15,1 is that it natively provided power management for IGPU and AMD GPUs.
A disadvantage is that it is based on 4th generation Haswell architecture and you need to provide
CPU power management. The architecture is closer in time than IMP1,1 for example.