Should I enable or disable Application Power Management and HPC?
As an overclocker myself, I would say No. Do not disable Application Power Management, and don't enable HPC Mode.
All disabling APM does really is cause your CPU to run outside the 125w TDP range. In essence, your drawing more power and voltage, and creating more heat for very little benefit. The same goes for HPC Mode. Unless your running some cluster server with very specific applications, enabling HPC mode is just going to generate more heat and power draw for very little benefit.
I can say 99% of CPU throttling problems on FX CPU's are due to either buggy BIOS in need of an update, or the more commonVRM throttling. Your motherboard will throttle the CPU if the VRM phases get to hot or outside of a safe zone coded in the BIOS. This is hard-coded in and set to help prevent frying your motherboard.
This is most common on the AMD 970 chipsets and the lower end boards that have 4+1 power phase designs. These are not high end power phase designs, and even an FX CPU at stock will throttle on these boards. Moving up to a 990FX board with a 6+2 or greater power phase design and good beefy heatsinks on the VRM and chipset will result in throttling problems going away without even having to change or mess with those BIOS settings.
In my experience, on an Asus M5A99FX Pro Rev 2.0 board, enabling and disabling APM and HPC Mode had no discernible effect whatsoever in any of the applications or games I run. I was still able to push 60 FPS solid in Crossfire on games such as Skyrim, Devil May Cry, Tomb Raider, and other titles with APM enabled and HPC Mode disabled. the only difference I saw between APM disable and HPC Enabled was higher core temps, high socket temps, and more power draw. The performance of the games and applications was identical.
The throttling many say these CPU's do, on the right motherboard with a 6+2 phase design or better, without buggy BIOS's are merely cosmetic. The FX 8350 dropping to 2.9 GHz for 1 millisecond or less before jumping back up to 4.2 GHz Turbo in game will not be noticeable whatsoever, as these drops are algorithm based to do so when the CPU has room to do so. There is more too these settings then just throttling, sometimes, it may just slightly drop Vcore while maintaining base clock rate to lower heat and keep the TDP profile, othertimes, it will boost voltage and multiplier to give you a boost.
The only time and situations I would recommend Disabling APM(Application Power Management), and enabling HPC Mode is if you have:
1. A very good preferably high end liquid cooling solution for your CPU for planned high overclocks in the 4.9 to 5GHz range that would go over the TDP limit anyways.
2. You have custom heat sinks and active fans on your motherboard's VRM, Northbridge, and other chips on your motherboard.
If your not overclocking that high and don't have the active cooling for your motherboard, I would err on the side of caution against it. If your having throttling issues, they are more then likely related to motherboard VRM throttling or buggy BIOS. In these cases, either invest in a better motherboard, or see if you can get your motherboard manufacturer to update the BIOS to fix the issue.