I think the limits ATI/AMD put on the cards are there because if you much higher then the clocks they allow, you'd probably be required to raise the gfx cards voltage. If they allowed us to do that then we would all have godly cards, as the gfx card range tends to be exactly the same just some yields can't clock so high are given as med/low range products.
Just a guess though, I don't follow the in's and out's of things that I generally can't change...