Its common sense.... I'll state it step by step for you though...
1) The cores are IDENTICAL. Stock clocks are different, but clock for clock identical.
2) Because they are identical, power consumption is going to be VERY close. +/- 15W.
3) Hence, clock for clock these hash identically.
4) The cooling solutions are designed for their TDP. One has a TDP of 375W and one at 525W.
What more do you want? Why would I spend $2k on these shit cards just to prove you wrong?
I don't understand how the power usage could be identical when the "unofficial" 7990s use three 8pin PCIX power connectors, and the new one only two? I had one of the powercolor ones, it said it needed a minimum powersupply of 850w. (It was right, BTW, it wouldn't start on a 750.) The new XFX one is 750?
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Because the maximum power delivery and the maximum heat dissipation are not the same as maximum power draw! They are 3 different things.
1) Maximum power delivery, how much power the infrastructure on the card can give to the chips. The old 7990s was 525W [75W PCI-E slot, 3x150W 8 pin PCI-E connectors] and the new ones 375W [75W PCI-E slot, 2x150W 8 pin PCI-E connectors]
2) Maximum heat dissipation. This is purely how much heat the cooling mechanism can remove from the chips, given certain ambient conditions. Old was 525W, new is 375W. Please note that while 1) and 2) match up in numbers, they are different variables. They are typically lined up because there is no point cooling more than the card could ever use [waste of material] and no point delivering more power than the card could ever cool [dangerous].
3) Maximum power draw. This is how much the chips could physically use. There is no point delivering more power, because the chips don't need that much. It would be like a pool spamming a billion packets at your miner when it only goes through 10 a minute.
I can see why you thought that the new cards were better because they 'used less power', but in fact they're worse because they can delivery and cool less power. The power draw (consumption) of the actual chips is the same - AMD is now just saving costs to reduce the overhead on the power delivery. The fatal flaw with this is at high computation loads, esp scypt, esp with overclocking, the power draw > power delivery.
And so the card power throttles.