I have used the performance seatpost rack with panniers for years without problems??? But I am also not loading them with 40 pounds of stuff.
http://www.performancebike.com/shop/...tegory_ID=2325
The best alternative that I have found for my other road frames is to use a Tubus Fly rack with their pclamp system or the lower bracket extenders that attach to the rear quick release. (Peter White cycles)