Doesn't the 20D start to hit diffraction around f/11?
If I was really obsessed with front to back sharpness I would definitely want to get one of the TS-E lenses. Set the lens to the sweet spot around f/5.6-f/8 and then use the tilt function to get the depth of field you want.
Well, that's how medium and large format landscape cameras work. I'm not sure the tilt lenses for 35mm digital backs work as well though. But it's a good point. At this point, after buying the Canon 17-35L, the 24-70L and the 70-200L IS, I'm not looking forward to yet another lens.
And by the way, the more you do full scape shots, the more you will become obsessed with front to back sharpness because you will become more obsessed with balance and synchronicity in your images, which in turn means you need foreground objects midway objects, and distant objects. Almost all top notch landscape images I've seen have this element to them. It's just the rule for me now. I mean even when you are shooting sand, sea and horizon, well, there you go--the three balancing objects--sand, leading the eye to the sea and the sea and reflection from the sun leading the eye to the horizon and probably the setting sun--just as an example.
But this is yet another topic, but you brought up a very interesting subject. Maybe you could start a thread? Perhaps I am dead wrong in this practice? And for sure, there are times when you can always break these rules and get an outstanding image. But tehn I would not know. I have yet to get an outstanding image--lol.