Really?
FSX is able to use 256 cores.

When scenery is loading I see all 8 cores pegged and at 5GHz.

As I understand it cores 1-7 handle scenery loading while core0 handles the games physics so it takes most of the heavy lifting.

On my 3770k system if I enable Hyperthreading it actually slows down whereas the AMD 9590 will use everything FSX throws at it.

I think it was the FSX sp1 patch that introduced multicore, it's not as good as it could be as Aces were closed down soon after.