The 3GB GTX 1060 was 13% slower than the 6GB model at 1080p when running the Warhammer benchmark in DX12. Using the older DX11 API meant that the 3GB model was just 6% slower, which is the kind of performance margin Nvidia suggested we would see.
At 1440p the 3GB model was 9% slower when testing in DX12 and 7% slower for DX11.
The Doom results are mixed. Testing with OpenGL we get the exact same 117fps average on the 3GB card as we did with the 6GB card. However, switching to Vulkan boosted performance of the 6GB model by 4fps, while the 3GB model saw a 30fps decline in performance. This is a strange result indeed and although 87fps is still playable, GTX 1060 3GB owners will want to stick with OpenGL for this title.
At 1440p we find similar results. The 3GB and 6GB versions of the GTX 1060 deliver similar performance when using OpenGL. However once Vulkan is enabled the 3GB card sees a serious decline in performance.