There is a great range in how humans actually do work, so there is no clear line between optimization/improvement and perfectability--after all, isn't "perfection" just another synonym for improvement?
No, "human perfectibility" is a Rousseauvian strain of explicitly atheistic utopianism which crops up a lot in modern philosophy political and otherwise.
At its core, it is precisely the Christian doctrine of the new life in the world to come, only with the serial numbers filed off, and God and the eschaton elided from the picture. As such, it makes no bloody sense whatsoever, but still has a strong appeal for Unitarians and others uncomfortable with religion and thus unavailing of the Christian utopianism of which "human perfectibility" is a corruption.
