I tried one xfx r9 280x but that one wouldn't even flash for some reason (flashing failed).
Flashing a 7970 with a 280X BIOS needs to be done from command prompt using "-f" (force option) because of different sub ids.
I pesonally did the same... I had 7970 Matrix and it didnt get over 520khash/s. After flashing it with BIOS of 280x I get 750.
Do not forget to set GPU clock to 1080Mhz and mem to 1500Mhz.