I think those cards hung 512 MB of RAM off of a secondary controller, so you can only access that pool on a 32-bit bus unlike the striping across the rest of the RAM, so it tanks performance above 3.5 GB.970 is 4GB GDDR5
I never owned a 970 so i'm not 100% sure on this, but sounds like something that would be reality.I think those cards hung 512 MB of RAM off of a secondary controller, so you can only access that pool on a 32-bit bus unlike the striping across the rest of the RAM, so it tanks performance above 3.5 GB.