Whatever the theoretical position on not obtaining any dual core benefits, when running FSX, I'm sure the l effects can't be dismissed so easilly.
Sitting with FSX and ASv6 running, with the Garmin GPS displaced to one screen and instrument panel to another, I've also got Task Manager open on one of the screens. For the majority of the time one core is used 100%, with the second showing about 10% activity. They combine to give 50 to 60% total useage. At other periods, possibly when ASv6 is downloading weather, the second core activity increases to about 20 to 60% and the combined useage rises to about 75 to 90%. Unless you're running just FSX, with no addons, no MP input, etc it seems the second core is definitely an advantage.