Should I buy a 200-500mm f5.6?

trolley

I'm currently using a 300mm f4 with a TCE14E-II on a D800 for bird photography.
I'm severely tempted to get a 200-500mm f5.6 for the extra reach (and the VR, given UK weather).
But I'm worried about the image quality difference. I used to use a Sigma 170-500 f5.6, and you can see the image soften before your eyes as you zoom in.
Obviously if I had any sense I'd trade in the 300mm in p/x for the 200-500, but would I regret it?
I need a 600mm f4 - but that's not gonna happen unless I get divorced

spraynpray

@trolley, I have moved this post to the other 200-500 thread.