24mm on FF is like 15mm on APS-C, so you're already pretty wide. The 17-40L would be the most likely candidate for UWA on FF, but if you shoot a lot wide open, you may want to consider the 16-35L II.

FWIW, I shoot a lot of birds/wildlife, and I haven't used the 7D since getting the 1D X, and I'm confident that would be true with a 5DIII as well. I'll be selling it soon...