Gigabyte GTX670 OCOC'd GTX 670 was never, under any circumstances,15% faster than stock GTX 680's.
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_670/31.html
5% faster than reference stock 680's. I see the point you're trying to make but let's not start exaggerating and making stuff up.
Stock=GTX680 performance.
OC to 1060Mhz=12%faster than GTX680.
Mine GTX670 was able 1280Mhz core.That was 15% faster than GTX680.

And best thing:Gigabyte GTX670 oc cost only 400USD and after oc it was way faster than GTX680.Imagine GTX1070 cost only 400USD and after OC beat GTX1080 by 15%.
Yep that good was GTX670.
https://www.techpowerup.com/reviews/Gigabyte/GeForce_GTX_670_Windforce/
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