it's not worth it IMO. i can still sleep well in C and would rather have a better meal in a restaurant before the flight and safe a lot of cash.1700 EUR get's you a nice case of fine wines that provides a higher satisfaction than what LH F is serving.
Of course LH's C class is not state of the art anymore but for 1700 Eur difference I'd take C over F anytime. Maybe i will think differently in 20 years from now.