Well, if your CPU works fine at 2.6GHz, it is not a dud. Overclocking is YMMV. I had three Opteron 144 chips. Two would do 2.7GHz+ and the third (actually first I got) would only do about 2.3GHz. People around here were telling me that I must be dumb or doing something wrong, or have the "wrong" hardware. I was testing all using Corsair XMS (which is another story) and a DFI Lanparty Ultra D, and all three were tested using same setup. However, some people just couldn't bring themselves to believe that "bad" overclocks could exist.
Reading over all these recent FX-55 threads, I see one other person only able to get around 2.7GHz stable. Most are around 2.8-2.9GHz. Very few at 3GHz+. One person was at 2.9GHz running Prime and the CPU died.
I guess the "better" cores are reserved for the FX-57.