Good day Damian,
Our CGAX-R9399 = Radeon R9 390 RoyalQueen is build with the same components, pcb and Tripple Fan CoolStream Cooler that we also use for the R9 390X.
This means that we provide better quality materials than necessary on the R9 390.
The memory is running at 6000Mhz already, and does not have “lot’s of room” to OC .
The GPU is clocked at 1010Mhz (where 1000Mhz is reference), i know some users have this running on 1075Mhz in a stable manner, but i suppose this also is depending a bit in the individual GPU …
Overall you should be able to get up to 10% extra performance with OC/finetuning the settings.