Curious if you considered the Luxon strut bar. Steel and no joints to deflect. I'm unaware of anyone doing deflection testing, but on visual examination it seems to be a stiffer design.
http://e89.zpost.com/forums/showthread.php?t=1247510. Under $200
shipped via eBay.
One other thing - we found binding issues with the stock rear end links and H&R bars. We went with a rod end link setup which greatly improved the rear end's behavior (which was twitchy prior). Anyway I should have the rod end part numbers so you can roll your own if you are interested. Just need to pull the info together and get it posted.
Thanks for posting ... let us know how it goes!
Filippo