Do you think that stretching wire is going to bend a 1/2 inch steel bolt, set 7 inches into concrete?
I don't think so.
Then you do not know much about stretching wire. Hell, I could pull the two foot deep cement base out of the ground stretching a corner with my little "Golden Rod" stretcher. Corner posts on a real fence go in the ground four feet deep, and are at least eight inches in diameter. Three feet deep will suffice if you tie two of them together with diagonals and tensioning strands as in an H-brace.
On the other hand, you do not need piano wire tight to keep a chicken fence upright. Still, I would put some diagonal bracing on the corners.
