I'm not sure exactly where the significant performance increase came from after I added another 8GB of ram, but it was very apparent. At the time of the upgrade ram was quite cheap so I decided to try it since I felt my computer's hardware was about maxed. Running FSX, weather program, ai traffic, high detail add on scenery, and using complex add on aircraft the improvement was noticeable in just about all phases of flight and ground taxiing. Some part/parts of my system are benefiting from the extra ram; I wish I had tried adding more ram sooner. Places and situations where I once had micro stutters are now very smooth. Large complex payware airports I will admit still see a large frame drop but not as bad. I was shocked at how large a difference it made to my sim. The reason I had never tried upgrading sooner was that I believed as you, that the extra ram would be a waste. I can't guarantee that you adding extra ram to your build will result in the same performance benefits, but I swear it was the best under $100 upgrade I have ever made to my rig. In fact if I had known the improvement it would make I would easily have given several extra $100's. Oh, and this improved performance was not seen after a fresh install or defrag; I just plugged it in for 4 4GB modules and started it back up. Totally new sim experience it was.
Matt