OCC has published a review on the PowerColor Devil 13 HD 7990

Managing the overclocks on a pair of cores is not much harder than overclocking a single core. The one thing to look out for is when one core crashes before the other, then working the overclock to work within the overclocking margins of the pair versus the single core. In essence this card was no harder to overclock than a single HD7970. The maximum stable speeds I could attain on this sample were 1137MHz on the core or 23% higher than the factory set 925MHz base profile and almost 14% higher than the 1000MHz Boost profile. Overclocking the 1500MHz rated memory that is clocked at 1375MHz from PowercColor yielded a decent 200MHz boost to 1575MHz or a close to 15% boost. Overclocking does deliver solid improvements across the board in each of the tests run. Improvement were most evident at a resolution of 5760x1080 where I saw an increase of 14% in CIV V, 25% in BF3, and just under 18% in Batman Arkham City.

  PowerColor Devil 13 HD 7990 Review