Pretty sure the 580 has better per core, per clock performance.
Think of it like bulldozer vs SB... I guess. A i5-2500k @ 3.3 GHz vs a Bulldozer cpu at 4.2GHz is slightly faster, it doesn't need to continue to match clock speeds at this level to continue being faster because it has higher IPC, meaning it gains more for increased MHz than bulldozer does. 7870 vs 580 works exactly the same way.
You told me not to compare CPU to GPU when I was just talking about pricing and profit, which since you make and sell both of them you can compare those metrics. Now you are trying to compare BD perf to SB and somehow make it the same as GCN to Fermi, which, except for the job they do, have nothing in common.
Going with your analogy though, if BD was 1/2 the size of SB used 2/3 the power to give the same performance, no one would care that it had more cores or ran at higher clocks. It would be a decisive win.
Fermi does not compete with SI, period! Which is fine, because it shouldn't. The only reason we are comparing them is because nVidia hasn't brought Kepler to market yet. nVidia will now have to play the pricing game until they release their new gen cards.