The DOF is insanely thin at 500-600mm - many of us have joked about the learning curve we needed for this lens... like getting BIF wing-tips in focus when they are close... lol
I came from 300mm where f/7.1 usually gave me wings in focus (herons, osprey, etc.)... and a shutter of 1250 was fine to freeze the flaps,... now compression and all these other physics variables come into play.
but you had 500mm so not so much a curve for you
Truthfully the 50-500 OS was more like...450-ish, its a reason i sent it back, Doing side by side with the 100-400 there was NOT a big difference between the two lenses..
This thing? Holy crap its long, I have had to back it off a bit a few times here or there to frame things right.. One reason i really like zooms for wildlife...
I will likely use it more at f/8 or so than f/6.3 unless i DESPERATELY need that extra 2/3 a stop..
Also debating on if i want the 7D Mark II or the 6D now...