IF AMD changed something significant inside their GPUs, it would have been mentioned in their reviews. At the moment, I believe their difference in performance in certain games/scenarios, is only due to different clocks (and perhaps more vRAM) and some "nvidia kepler" move in regards to the drivers.
Exactly. If the 300 series had GCN 1.2 then I'm sure AMD would have advertised that instead of keeping quite.
Not sure about gimping drivers though. My 7900 cards have been goods so far and have had good fps in Witcher 3, Farcry4 etc without using modded drivers. I think AMD may have genuinely got some improvements in the newer drivers due to rewrites or learning a few things from the Windows 10 driver code.