For downloads there isn't any difference between 5.1 & 5.2 in socket code, when bw limits are off (in non-ssl mode). Upload code differs (everything goes through encryption thread), but that shouldn't have any (atleast noticable) effect on performance.
What you might want to check is, that you get "right" ports as PASV reply, and that active mode (PORT) is actually binding to port - 1.
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