Looking at the review of the Sapphire 5700 (they did the basic as well as the XT), on Hexus - yes they can be overclocked but its only a very small amount. Also basically no basic performance gain vs AMD's things from the OEM cards. Much better cooling all round of course, so no real competition.
So the not insane expectation of being able to manage to force a 5700 to nearly match an XT via overclocking/third parties seemingly didn't work out for whatever reason.