Well i don't follow that outside the box thinking. Get a 7d when you have a 5dmk3? So with the 7d he'll get twice the DOF, thats not going to help isolating people in sporting situations. Then he'll be demoted to 18mp and lose, what, 2 stops of ISO performance, and get a much weaker AF system, less spiffy LCD etc. If you have a 300mm and a 5dmk3 and you think you would like a longer setup occasionally, I would get the 1.4 tele and if that doesn't get it done, crop. Or just crop now and buy nothing. I get amazing prints from my 13mp original 5d. so with my newer mk3 with 23mp to burn i see that as a built-in 2x ext when neeed This setup isn't to far removed from mine and I've got no complaints. I run the mk3 with a 300is2.8 and sometimes slap a 1.4 ext on it. Unless of course you really need 23mp files for some reason, or want to purchase, learn, and haul around another camera body, with a back up battery.