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I would ring them and tell them whats happening and see if they can suggest anything.
Is the problem the same all the time, or is it only when the lens is wide open?
Tried it on a tripod with the mirror locked up?
Might be worth doing some side by sides on a tripod with your 50 and seeing if the differences change or if you can reproduce the problem at certain settings.
The more information you can give Sigma the better imo.
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