> So the correct result for b'\xff\n'.decode('gb2312', 'replace') is u'?\n'?

I think it should be so. This behavior does not leave out possible information, has no side-effect on later decodings, and should the '\n'  indeed be redundant, an output of u'?\n' would unlikely cause confusions.

Though, I have no knowledge on this subject code-wise. If a change of the behavior will have an impact on performance, maybe the change should not come in.