I have used a 5D3 but not at high ISO but much prefer the 7D2 in general for sports. I own a 7D2 so I can tell you that personally I have no problems going up far higher than 1600, I shot a rugby game this weekend and ended up at 6400, the shots were very much useable. I think people get confused when they are shooting things in dark conditions, they under expose things, then blame the ISO when they have a crazily noisy shot.
I guess it's a bit subjective though, what I think is fine and useable, you might not.
If you're seriously thinking of buying one, why not hire one for a day and test it at an event along side your 5D3, if you then buy it they might knock the days hire off the cost. If not, worth the cost of hire for one day to avoid buying a camera you wouldn't be happy with.