I think there is a misconception about the charts. Basically, you may not operate outside the limitations of the aircraft and/or extrapolate beyond the performance limits of your aircraft's performance charts. This means if you have 50 knots of headwind, you can only use 15 or 20, depending on which charts you have. Similarly, if you operate below sea level, you may not claim any benefit for doing so. However, if you have a temperature above 40 Celsius and that it where your charts stop then it's time for a cold beer. Yes, people do fly beyond the performance limits of their graphs and some fly after a wine or five. But both are still illegal.
PM