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Ive heard/read a lot about lever throw on Shimano drop bar discs but have never worked on them myself so I can but guess.
Seems like there's something wrong with the way Shimano have translated mtb levers into road ones.
I imagine the problem is at the lever as it's the bit that's different. Ergo, new calipers aren't going to solve your issues.
I know you say mechanicals can >>>>>>> but have you tried a bb7?
Drop bar levers?