Oh hai!
Yeah, I installed my Progress rear sway bar about 4 years ago. It was good for about a year. I lived in an apartment at the time with almost a curb to drive up to get into the parking lot. I used to angle it so I wouldn't scrape (and I'm not even that low) and one day the twist was finally too much for the rear sway and it broke the passenger side bracket. I emailed Progress with some pics and asking for a new bracket and they shipped me a new one free of charge and told me to be careful when torquing them down, go little by little and alternate. So when I installed the new bracket I did just that and torqued them down with my impact. That was about 2 years ago and I haven't had problems since, even with tri-podding on the BC coilovers.
I think they clunk because the bar will hit the rear control arm (or whatever it's called). It's really not a big deal and doesn't hurt anything, but the clunking does drive some people batty. I never really cared so I never looked into a solution so I can't really help out with the clunk fixes. But for some reason I don't have a clunk anymore with the coilovers. Or if I do I just don't notice???