Like Nikons version I expect this lens to be approaching the size and weight of a 500/4. The main advantage is reach, 560 f5.6 with the converter over 400 f5.6. I have both a 300/4 and a 500/4. To my eyes they are both about the same optically, even with teleconverters, but it's worth lugging the 500 around purely because of it's reach. Every time I don't I regret it.
With frame fillers and fieldcraft, that's right to an extent, but in my opinion it's also very important where you actually are. For example in the birdguides POTY, they had a survey, and most of the notable and POTW where taken in either Norfolk, Suffolk, Linconshire, or Lancashire. I remember when I once visited Salthouse in Norfolk, you could walk right up to Dunlin, Snow Buntings, Teal, and Little Grebes, but this is not the case where I usually go. In the London area, Teal, Shovelers and Little Grebes are very shy and difficult.
My local reserves are Stockers Lake and Staines reservoir around the London area, and I would say for both that reach is king.