Apparently, the second Gigabyte GTX 670 needs to be RMA'd. It eventually started randomly blacking the screen out from time to time and crash during heavy gaming. At one point it couldn't even open any game or any program that requires a heavy graphical load. I though it was just my 80 Plus Bronze low end GS800 PSU so I ordered an HX1050. While waiting, I swapped the second GTX 670 up to the x16 slot by itself. So it turned out that the second GTX 670 was an unstable POS even when flashing to an updated BIOS.
I am currently back to a single GTX 670 until the RMA process goes through. But in the meantime, here are some Unigine benchmark comparisons from single to SLI at 1228MHz core clock and 3305MHz memory clock, before GPU went to s**t.
Unigine Heaven 3.0 single
Unigine Heaven 3.0 SLI
Unigine Valley 1.0 single
Unigine Valley 1.0 SLI