PC-E - PC = Perspective Control, E = electronic diaphragm control (the camera stops down the lens electronically, even though the aperture is set manually on the lens)
Really? What I've shot on my cropped sensor looks OK at 2.8. I've mostly used it on film however, and I think it's plenty sharp enough for 35mm slides. What have you noticed wide open?
I'm spoiled. I shoot mostly with the 85 1.4, so when I look at 20mm at 100% on the screen at 2.8 I'm not happy. However, stopping down a bit works just fine. On the shots above, I could not tell the difference in sharpness between the 20mm and the 24mm PCE.
None at all. HDR takes care of the deep shadows. In the 20mm shot, the toilet area had almost no light at all since the overhead was not working properly.