I completely disagree. Couldn't disagree more in fact.
So here is a comparison of a 580 a 680 and a Titan with a chart of the FPS achieved in a good number of benchmarks and in-sim testing (FSX & P3D). This clearly shows that the GPU can make a significant difference to achievable frame rates and correlates to my own findings in FSX when I upgraded my graphics card (overall 40% increase in FPS).