With the lenses you have, I don't see think the Sigma 14-24 f/2.8 has much value to you. It gains you one stop compared to your 11-24 -- that is pretty much it. I don't think another lens for 1 stop is worth it especially for landscape applications. I can see the Sigma 14 f/1.8 having more value as an astro only lens because it is 2+ stops faster, but it's use case will be limited. I'd rather cover the focal length range first and then think about max aperture.
If you are considering the Sigma 14-24 f/2.8, then I'd also strongly consider the Canon 16-35 f/2.8 III. It accepts filters and has a wider focal length range that is more applicable to general usage. When given the choice, I opt for the 16-35 f/2.8 III over the 11-24. Unless I know I need the 11-15mm range, I find the 16-35 f/2.8 III easier to use. Easier to protect with hoods, faster aperture, lighter and not as front heavy, fits better in camera bags, etc.