" Ive got the 500f4 mk2, both mk3 extenders, the 70-200 2.8 but theres a big gap in my range, around 300mm."
I'd like to share my experience on the above issue... I do birds and wildlife and a little sport so I am not that well versed on sports. For wildlife applications, I am not sure what a 300 is good for if you have a 500mm.
I have a 300 f/2.8 (version I) and traded in my 500mm (version I) for 600mm f/4 II. The truth of the matter is that I have used the 300 f/2.8 more or less once a year. Once one commits to travel or hike around the trail or the marsh with a 500-600, you do not want to haul a bonus 300 f/2.8. Or try to pack gear in a carry on with both a 600mm and a 300mm. At the end of the day, it's very unpleasant to move around or travel with both a 500/600 and a 300 f/2.8. Maybe it's workable around a football field, although this would not be my choice; I'd commit to one or the other.
So if one is not going to carry both on site, one must choose. Is it going to be 300mm or 500mm? The 300mm will not win this very often when it comes to wildlife. There will be more opportunities with a longer lens, if action gets to close, there is portraits opportunity. If it gets to far, oh well, too bad.
Last summer, I went to Katmai Nat Park to photograph Grizzly bears catching salmon on the river. This is a float plane access kind of place, so weight is a concern. 'There is a viewing platform that can get pretty close to the bears and the 300mm would work great on those. (Len Rue Jr was there with a 300mm, so this is certainly not a bad idea). But on some other viewing locations, the bears are pretty far. After weighting this out, I settled on bringing the 500 + the 70-200 f/2.8 II with both converters. I used a 5D and a 1D mk IV with both lenses, with gives even more framing options with the 1.3 crop and FF. I used the 1.4 III converter heavily at close range, mostly with the 1D m IV. That did the trick in the falls and I brought back nice shots of bears catching salmon with the 70-200 (Not as good as the Mangelson classic, but pretty good :-)). This is the only time I considered the 300 over the 500 last year for a trip and I am glad I did not bring it. When shooting wildlife, we end up adding the 1.4 or even the 2X *a lot* to a 500 or a 600. It's sure possible to shoot the 300 with the converters and get to 600 with some decent quality, but this is where it stops.
Next weekend, I will be heading to Sacarmento NWR to spend some quality time in a photo cache in a swamp. Last time I was there, I had the 500 with the 2X on when a peregrine falcon perched close by. I ended up with to tight head shots and the bird flew away when try removing the converter. This year, I will have the 300 and the 600 ready to shoot. I am getting some use for the 300, but in all honestly I could do without.
What needs to happen for wildlife - Canon needs to ship this elusive 200-400. Depending on the application, I could leave the 600 behind. One of my fellow nikon photographer brought the 200-400 to Katmai last summer and did well (with a D800). This was the first time in a while I had Nikon equipment envy. I hope to sell my 300 f2.8, an old bimmer and other few trinkets to help finance the 200-400 when it comes out.
In short - as a bird and wildlife photographer, I can easily live without 300. If you must have one, you have to look at a f/2.8 - I am not sure why you would need a 300 f/4 with the 70-200 and the 1.4x, no one will see the difference (I know it's 20mm short). Canon 300mm f/2.8 version I are a good deal.
One last note - one good reason to bring a 300 over a 500/600 or a future 200-400 in the field and weight and size. The difference between the 2 outfits is huge.
Good luck with all the choices,
