The Sigma 35 is indeed very sharp. However, I tested it carefully against my old Canon 24-105 L at 35mm, using an iso 12233 chart. The 24-105 is sharper (at f/4 on both)! This is the second time the old workhorse has outshone the new pretenders (last time a Tamron 24-70 VC).
The Sigma 35 A is reported by Photozone and lenstip as having significantly superior MTFs to the 24-105. Either I have been unlucky with the Sigma and Tamron and getting poor copies or I have been very lucky and have a 24-105 which is an outlier in the right direction.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye