Lots of money for little gain. I would dig proper holes with a post hole digger, put at least 3' of the post in the ground, backfill with the dirt that came out of the hole and tamp around the post. A typical large pry bar has a flat 2" button on the opposite end of the point, and makes an excellent tool for tamping. This is how, literally countless millions of posts are set, and it works just fine. IMHO, you are best of avoiding any concrete in the holes. Good luck.