From various reports of business partners on these routes (FRA-SFO-FRA and MUC-SFO-MUC) it seems C and F are always packed to the limit, whereas there was always some free seats in Y. Did result for some of my business partners in voluntary downgrade offers from C to Y for a lot of cash.

When looking on the F0 and C9 situation, keep in mind the "Buy C and Fly F" US/CAN offer is still in place. It is not a bad deal to get an F flight for about 40% below the regular F fare.