While this is a slight tangent to the OP's question, I think international F on a 3-class AAirplane is worth it if the flight is over 7-8 hours and if it is an award booking. Off-peak one-way Y fare SFO-LAX-LHR is 20k miles. Business sAAver is 50k. First sAAver is 62.5k. If one is going to burn 50k miles one way in C, I would definitely consider spending 25% more miles ("only" 12.5k) to sit in F.
If spending hard cash, I most likely would not spend extra money or miles to upgrade to F. I might consider using an eVIP to F for the PVG or NRT flights though.