I would forget about the sigmonster.
I would forget the 600mm since you need dual sports and widlife duty (although for wildlife only, it would be the one for sure, but for sports it will be kinda long for some stuff and not flexible enough as a one lens for all solution).
So that leaves 400 2.8 (+TC for wildlife). Or 300 2.8 + TC 1.4x + TC 2 set if you are more value and light weight
Or 200-400, it's only f/4 but that still might be ok, depending.
none of these are exactly value conscious
all the canon's take tc 1.4x very well and they handle 2x ok too although you won't get full on magic IQ with the 2x on, the new II might handle it a bit better, but you can still match like a tamron 70-300 wide open sort of quality with the 2x tc on the amrk i super tele, which is more than usable
I am thinking about a long L series telephoto. My interest is action sports and birding. I am a value conscious buyer.
Canon EF 600mm f/4L IS II USM
Canon EF 400mm f/2.8L IS II USM (+teleconv)
Canon EF 200-400 f/4L IS 1.4X (when it becomes available)
OR Sigma 300-800mm f/5.6 EX DG APO IF HSM (sigmonster)
I am looking for long term use and value in the purchase. I know at least one of the items in the list has not been released yet but apparently there are lot of people previewing it. How does the Sigmonster compare with the Canon L brethren? Considering that its about half the price (well OK not quite half), both from a usage standpoint (speed of focusing, weight, zoom etc) and from IQ. Which one of the Canon is the best choice, Would the 400 F2.8 + 1.4X Converter (not the zoom) be a near match for the 600mm F4? Because if its close, then the 400 F2.8 may be the better choice as it give you the 400mm F2.8 which is a sweet combo for many options, including sports. Whats your opinion? Especially if you'd had any experience with these guys? I plan to use it with a 5DMkIII in the first instance?