There are exisiting legal interpretations on this subject. Do a google search for "FAA Legal Interpretations". You will find a public, searchable web site.

If said headwinds were stronger than originally forecast, that "could" be interpreted as "circumstances beyond the control of the certificate holder". In this case, if you were legal to start, you were legal to finish. However, if the extra flyng time causes PIC fatigue, and the PIC does not discontinue the flight due to this fatigue, then this could be a careless operation and therefore may be found contrary to 14 CFR 91.13...