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I'm just curious, why are you putting this effort in it when there are dozens reviews online?
You are correct that two x16 lanes will give the best performance. However, all multi GPU will only ever have the need for dual x8 lanes, for now. The performance difference between two x16 lanes and two x8 lanes is very minimal, at best. Something like 1600 CL9 RAM vs 1866 CL9 RAM.
A motherboard running at x8/x8 is considered optimal without breaking the bank for a minimal performance increase.
No, no OC going on and I think my machine could do with a re-build.