DXO just published their measurement for the Sigma 35 1.4 and it set the highest resolution score among lenses with 35mm focal length, far surpassing the Nikon 35 1.4 and the Carl Zeiss Distagon 35 1.4 ZF2.
DXO also compared the Sigma 35 1.4 on a D800 against the Canon 35 1.4L on a 5d3. In that comparison it would seem that the Sigma's 23 P-Mpix ("perceptual megapixels") is almost double that of the Canon's 14 P-Mpix. However, part of that difference is attributable to the body. If our goal is to evaluate the lens' capability (not that of the lens-camera combination) then I think it would be more fair to use the same body.
Here is a DXO chart with obth lenses on a Canon 5d2 (also included is the Canon 35 f/2 IS):
On this test, the Sigma 35's resolution is still superior to that of the 35 1.4L but the difference is not as great as the previous comparison would suggest (17P-Mpix vs. 14 P-Mpix). In addition, the Canon 35 f/2 IS is able to match the resolution (and even the very low CA) of the Sigma 35 1.4, although of course the Sigma has a one-stop advantage.
Another thing that some have been wondering about is how the Sigma 35 1.4 compares against the Sony RX1's very well regarded 35mm f/2 lens. DXO's chart shows that the Sigma on a D800 has far greater resolution than the RX1. However, I think it is better to compare with a more similar body. DXO doesn't have measurements for the Sigma 35 1.4 on a D600, but it does have a measurement for the D3x, which I think is more similar to the RX1 (both have 24mp).
At last, the answer to the question of the Sigma 35 1.4 vs. the RX1: we find that the Sigma on a D3x out-resolves the RX1, although not by a large margin. (The D600 results are probably the same or a little better than the D3x.) In addition the Sigma has a 1-stop advantage.