I have an FX-8320 @ 4.3ghz, I've actually found I get best results after removing AffinityMask from my FSX.cfg. I have acceleration installed.
As Luke says you will really only see one core being used most of the time and the others picking up either when loading textures or to support things like AI aircraft.
In my BIOS I have disabled everything that will reduce CPU power usage or reduce speed. I also make sure my Windows power profile is set to high performance.
I've found the best test to see what cores are being used is to pan the view around the virtual cockpit of a complex 3rd party aircraft - for me the IRIS PC-21 flying over Orbx PNW demo area pushes up CPU usage on all cores doing that. My Realair Legacy and Duke Turbine use less and I usually see better frame rates when flying them.
As you have quite an overclock set have you checked it with Intelburntest and Prime95 for stability and watched to check your frequency and temps? It is not uncommon to have an overclock that is not as stable as you think but without blue screening or crashing.