I never owned any of the lenses you mention, but i'm looking for a landscape prime myself, so i can share the results of my research with you.
MTF charts show that the 17-40 is good enough when stopped down to f/8-11, so i think you're fine with it when shooting landscapes and long exposures. However, if you want take pictures of stars you're going to need a fast aperture and decent sharpness. The 24/1.4L is sharp.in the center, but somewhat disappointing in the corners when used wide open, showing low sharpness and high vignetting. It also shows coma when shot wide open. Stopping down to f/2.8-4 improve things a lot, but if you're stopping down to f/2.8, you might as well save a grand and get a 24/2.8 IS that boasts decent performances right from the fastest aperture. If you need the fast aperture for creative purposes, nothing can touch the canon L.
There's a brand now Zeiss 15/2.8 that's getting lots of praising reviews, but it costs something in the whereabouts of 3000$. The other Zeiss lenses show good sharpness in lab tests, so they are worth a thought, if you can get over the fact they don't have the autofocus.
My considerations: the 24/2.8 IS is a good all-arounder, delivering good performances in all the fields of interests at a fair price. If money wasn't a concern, i would have bought the Zeiss 15/2.8 as a dedicated landscape/starfield lens, and something else for street and handheld low light photography.