Also from maximumpc:
Key word is "compete" when we're not in Mantle mode. Good enough, but without Mantle it may not be a GTX 780 killer.
According to Nekechuck, even though the R9 290X uses a 438 square mm die, which is significantly smaller than the Titan’s 550 sq. mm GK110 offering, it “will definitely compete with the GTX 780 and Titan.”
Key word is "compete" when we're not in Mantle mode. Good enough, but without Mantle it may not be a GTX 780 killer.