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SLR Gear has reviewed the Sigma 300mm f/2.8 EX DG HSM APO lens and you can read their analysis here. The lens scored brownie points image quality as the it turned out to be sharp, and the reviewer found the distortion was negligible and light falloff almost non-existent.
Says the reviewer:
"Our tests show the Sigma 300mm ƒ/2.8 to perform very well: sharpness is very good, CA is optimized for use at ƒ/2.8, there's virtually no distortion and light falloff is almost non-existent. Optically, the lens stands well on its own".