Sigma 300-800mm/f5.6 or Canon 500/f4 600/f4??

I am mainly into bird and wildlife photography. I use the Sigma 50-500 today and I find it a bit soft. I also want to use the 1.4x and 2.0x converters (that normaly gives bad result with the 50-500). How "bad" results is there with the 300-800mm with converters?

I have searched for tests on the web without results. I would be more then happy if you can help me make up my mind.