Ditto to what most have said here. That just seems like crazy overkill to go more than 1/3 of the total post height, especially if you plan to 'crete them.
I can't comment on how well the coop stays put yet, cuz I'm not done (
), but my chicken run is a modified dog run I built 4 years ago. I used 8' farm fence posts (Those greenish rounds posts, about 5" diam), buried 'em about 2 1/2 feet, and I only 'creted the two posts holding the gate. The rest are just in the dirt, and it would take a backhoe to move them. The run is 10' X 24' with 2X6's at the top and bottom, and those posts haven't budged.
As for my coop, it's 8x8 and sitting on block over sand and I plan to anchor it with the wire and torpedo type anchors used to tie down swing sets, kennels and the like, just in case we decide to move the kennel in the future.
Oh, and I live about 8 miles from the Canadian border over here in Central Washington, so our frost line is just over 4 ft but even our cabin is only sitting on concrete piers 36" deep, according to WA code.
Good Luck, whatever you decide. And of course, we'll be standing by for pics!