High speed sync? My 5d mkiii also syncs at 1/200..., but that isn't really germane.
Sure I'd like a 400mm f2.8L is mkii, but I don't have a spare $12,000... though I can check the couch cushions... let me check right now... nope only $8,000, mostly in pennies. I have mulled over a 1dx, but I'm quite happy with the mkiii at the moment. Actually, when I shoot sports I time and anticipate... I haven't sprayed and prayed in quite a while.
Good news 400mm f2.8L is mk II down to a rock bottom $10,500!
Actually I was going to mention the 300mm f4 is a hell of a lens for the money. 300 2.8 is nice, but f4 will get the job done.
I did notice... and I'm going to be perpetually tempted until I finally dive into the pool. I considered the 300 f/4... but I have the 70-200mm f/2.8L is mkii plus the 1.4 extender. which gives me the 100mm to 280mm range @ f/4 (though admittedly with a hit to image quality).
Right now I'm buying a few accessories I otherwise am too cheap to buy, monitors (and evidently the color correction system), a good monopod, etc. After all in said and done... I'm going to be a good 6 grand away from a new 400 f/2.8... but maybe a used one will be only 3 or 4K away... and that isn't impossible... though admittedly... not likely.