The 4770K is a great processor for FSX, as it's one of a handful of titles which is multithreaded, and can take advantage of Hyperthreading, which improves minimum frame rate.
As you may know, unlike "gaming", FSX is a "simulation" which is 80% CPU and only 20% GPU, so your 690 is WAY overkill for FSX. A 660 is more than adequate. No matter how much GPU horsepower you throw at it, FSX has to be tuned to your CPU, so a high overclock yields the best frame rate.
Is your CPU overclocked?
I run FSX and X-Plane frequently. My 4770K is at 4.7 with a GTX 770, and I can't "max out" the settings, nor am I aware of anyone who can. I do know that memory timings, among other factors, will cause FSX to crash.
It would be helpful if you would list your complete system spec's.
CT