DXO Mark rates the Sigma 18-35mm F/1.8 as (T1.8) and the Canon EF 24mm F/1.4L II as (T1.6) so I'm thinking great, not a huge difference, just 1/3 Stop so perhaps I can sell the Canon 24mm.
Wrong - The Canon EF 24mm F/1.4L II is much brighter than the Sigma 18-35mm on my Canon 60D or Blackmagic Cinema Camera. Easily more than a full stop. In fact the Canon 24mm is still brighter at F/2 than the Sigma 18-35mm at F/1.8
The Sigma 18-35mm is a great lens, but don't be fooled into thinking it can replace an F/1.4 lens in low light situations.