I have some at 12~13 feet on a purlin and another row a couple feet higher but dropped from the purlin it may be a bit better. I have another row on a purlin at about 20 feet from the floor, the light is noticeably duller. The low row is primarily over my machines, the next row over bench work and fab, the high row is above a bridge crane(not much room to drop). The overall light quality is good, the high row add some fill that is reflected down.
I used all thread to drop hex boxes for my lights, it was not a big deal and I would do it again. When I did them all I could find was 3 foot pieces of all thread. You may want to line up your materials, I would have preferred 4 feet.
On the other side of my shop I have 55w's attached to the bottom of another bridge crane, under the rail at about 12 feet, even being 12 feet from the ceiling they do provide some good reflected light.
Steve