Of course you gain by going from 400 to 600mm - I have done it. Your arguments jrista are qualitative as to the amount of gain and are hand waving that it goes up by the focal length squared. But, the physics and maths quantitatively show it varies linearly with length. Let us just agree that it is a great improvement.
I went up to 600mm by using a 300mm f/2.8 II +2xTC III. It may not be quite as good as the native, but it is good enough and so light that I can hold it in my elderly hand for hours.
Here is a 100% crop of a heron 50-60 metres away I took on Sunday.
Really nice heron shot, Alan.