Its not a GPU limited game in the slightest, never has been. Note the amount of people that mention how cool their cards are when running FSX, also try reducing GPU limiting features like resolution and you'll notice no difference. Running it on my old 7900SLI setup there was no change in frame rate from the default of 800x600 to 2560x1600, absolutely none.
The glare option + the autogen have the greatest effect on performance and my 2 8800GTX's give me no better fps at the same settings I'm afraid, given how much more powerful these are than the SLI 7900's I had before, if it was GPU limited you'd see a change.