I'm use a VTX 7990 (I think is the same as powercolor). 4x7990 hashing ~ 4600 Mh/s in scrypt with this setings: core 835 @ mem 1375. My first 4 cores are undervolted to 0.925 and second 4 is default voltage (not accesible at bios).
Not trying to take this thread off topic, but I'm looking at building a similar setup with 4 * 7990. What OS are you using ? How are you cooling them ?