Is the HD7950 (R9 280) as good as the HD 7950 Mac Edition?
Depends. If you pick the right card (e.g. has reference port layout), it may perform better than the Mac Edition card (after flash) because has better cooler.
But if you pick the wrong card, then some port may not work after flash (but that port can be recovered by booting from the original PC ROM, most 7950 has dual ROM design, so you can always keep one of the ROM with the stock PC ROM).
Also, most after market card has higher clock frequency. So, more power hunger. And come with a 6+8pin design. Even though it's technically outside the cMP's limitation, but the card can be driven by internal power only without any issues, all you need is a mini 6->8pin cable.
On the other hand, the Mac Edition card come with dual 6pin design, so always stay within the official limit, and shipped with all the required cables.
Anyway, MVC's GPU is always a good choice if you don't want to do it by yourself.
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Once the card is flashed, you're Mac won't know the difference. It's the same hardware chipwise.
Apart from the flashed card can only run at 2.5GT/s without R17 resistor removal. Even though there is no real world performance difference, but they are not entirely identical (in the system point of view).
