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I tested the 15mm Zeiss and my 14mm L II is as sharp all over the frame. The only downside of the Canon is that it has more CA, specially closer to the borders of the frame. the 15mm Zeiss has little to none. That is impressive. The 14-24mm was also very good and very close to the Zeiss. But the Zeiss is easier to filter (no hood version available). No need for those huge 150mm filters.
I had the Canon 14L II. Excellent lens. However, it has a distinct field curvature, something that is not as pronounced on the Zeiss. And as you say, the Zeiss is extremely well corrected for aberrations. Another big plus of the Zeiss is the smoothness of the focus ring and the hard infinity stop which can be crucial when shooting the skies in pitch darkness.