http://forums.guru3d.com/showthread.php?t=396856 I would only advise you to do this if you know what you are doing and your card has a dual bios. Otherwise i would not recommend giving this a go.
Is only possible if your 7950 uses AFR memories, if your card uses MFR a BIOS from a 280 can cause 2D artifacts or don't even work. Check with this tool https://www.techpowerup.com/downloads/2326/asus-radeon-memoryinfo-1-005/mirrors
Not directly but a bit related: i was able to successfully flash hd7790 into r7 260x, here is my thread on it: http://forums.guru3d.com/showthread.php?t=389714 This gives some hope for you.
Sorry to hijack the thread is it possible to flash a 7970m to R9? I have this card modded on an asus g73jh laptop, and generally to make it work you need vbios which support the hm55 chipset. However I have installed a switch on the bios eeprom so I can blind flash when I f**k up
i flashed my 7970 to a 280x But i made sure everything matched up in both bios before i did it. and also tested the clock speeds