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the problem you are facing is that 7d + 400 5.6 is as good as you're gonna get for wildlife without breaking the bank. 5diii and 70-200 are not gonna help you get more shots of animals than you do now. mark iv and 300mm is a great setup, but it is gonna be MUCH heavier (~ 8lbs) than your current one; it might be so heavy you need a tripod. if jeffery is talking about the IS version of this lens, its gonna be 3.5-4k on its own, as will the mark iv. so yes you get a great rig, but you're still not that much closer to the animals and you've now put out at least 7k. this rig might be ok if you are shooting big animals, but it is NOT going to cut it for birds and small animals. however, a 500mm would help you get more pixels onto your subject and would be faster (f/4) and longer. then you slap on the teleconverter for a 700/5.6 which on the 7d is more than enough reach for all but the most spooky beasts. yes, a used 500 is going for 6000 bucks now, but 1) its an awesome lens and 2) its a cheaper plane than the 300/mark iv idea. here are a few samples of the 500/7d combo. these birds would have all flown off had i tried to get closer as the use of a 300 (or maybe even 400) would have required.
Canon EOS 7D EF500mm f/4L IS USM lens 500mm f/8.0 1/125s 400 ISO -0.3 EV
Canon EOS 7D EF500mm f/4L IS USM lens 500mm f/8.0 1/400s 100 ISO 0.0 EV
Canon EOS 7D 700mm f/8.0 1/1000s 100 ISO 0.0 EV
Canon EOS 7D 700mm f/5.6 1/60s 400 ISO 0.0 EV