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Some of the corners are cropped due to stitching but in general I find they're pretty good, even wide open. I also use most of my lenses on a Canon 6D so I'm in the process of making more comparisons. I do see an odd effect where the very center has higher resolution than the rest of the frame, even stopped down - but it's more complex than just 'bad corners'. I think some (not all) of the image quality limitations attributed to the Speedbooster are really just lens limitations.
I expected the central higher resolution boost from my Zhongyi FD Lens Turbo, but have also been pleasantly surprised by good edge (if not extreme corner) performance from my FDs. Obviously the lens quality has a lot to do with this! But the one-size-fits-all approach of the LT/SB can surely not produce the same performance on every lens, regardless of each lens' unique field curvature, distortion etc? Anyway, the edges of this FDn 50/1.4 + LT shot at around f2.8 (un-cropped, as far as I remember) are fantastic, especially as I wasn't expecting or hoping for them to be this good! Posting large, just to look at sharpness (embarrasingly, I've only just noticed sharpening haloes and green fringing. Must revisit this one!):
Dragon Boat Challenge race, Cork by specularist, on Flickr