Perhaps Sigma will be out with their new 120-300mm f/2.8 OS Sport lens before long and throw in a 2x converter with it.What's this mean? I've had that lens combination since it came out. Do miss the focus limiter switch of the new model but don't miss the price. Only thing I don't like is the inherent clumsiness of detaching a 2X TC, and the rather offputting OOF blur performance in some shots.
Other than that, though, sure 400mm f/2.8 would be a grand improvement but I actually do often find the zoom useful at times in wildlife shooting (usually going from extreme close - i.e. roughly 240mm before factoring in crop factor - to extreme long, roughly 560mm again before the crop), and the weight of the 120-300mm + EF 2X Extender III is already close enough to unmanageable handheld that I would think twice about going to a 400mm. Of course I don't spend hours sitting in blinds; I just walk all over the place and try to get closeups of critters. Works surprisingly well on many things.
I had a Sigma 120-300 OS and I've found it very lacking when compared to Canon's offerings. I ditched it for a 400mm f2.8 L IS and there is no comparison. Like many Sigma's with HSM, I found the AF to be a little erratic and a bit hit and miss for fast moving objects. It was too large and heavy for what it was. The focal length was well short of the stated 300mm, nearer 280mm at infinity. Focus 3m away and it's nearer 240mm....which was appalling. If I popped a 2x converter on it and A-B compared focal lengths with my 400mm at 4m , I found them to be very close in focal length....which means where guys are thinking they have a 600mm f5.6, it's actually closer to a 400mm f5.6. This focal length is better realized in a number of options...even a 2x on a 70-200 f2.8 II L offers a better focussing, better IS, lighter, smaller and cheaper options. Optically there was little between them (70-200 vs 120-300) but my 400L is better all round except weight. Even without a converter, at it's closest focus distance, the sigma gains only 40mm over the Canon 70-200mm f2.8 L IS II....which is a tiny increase at such a long length. I think this Sigma lens is a missed opportunity and that's a real pity.
I seem to be one of the few who could really give a stuff about the new 200-400 f4 L IS lens. If I had the money, I'd either replace my 400 L IS mk I and upgrade to a mkII or consider the new 500mm f4 L IS II as my light and portable option....have you tried one? It's SO bonkers light!