I've made comparisons with my xf14mm software corrected. The difference is small, you really have to pixel peep. But you lose resolution: with the 14mm, the same scene, from the same distance, perspective corrected has to be cropped and ends up roughly as a 3069 x 4603 px image vs 4000 x 6000 px with the 15mm shifted. (around 23% less linear resolution)
As regards to the perfect perpendicular correction I agree completely but this was a test, and I wanted to see the distortion of the lens, I find it's easier to see if aligned perfectly, you can then compare to the edge of the image. The image I posted has no distortion correction.
here is the same scene with the 14mm corrected in lightroom: