Where did you get those ranks? My F60 is at max pro and it has the ranks you replaced. (1637, 1793). If you're attempting its R&D, the event doesn't allow you to fully upgrade the F60 after the first test.
I don't know. But I noticed there are still a lot of inconsistencies in the S class relatively to the +TK bonus. In D class the +TK adds an increasing number (+39, +40, etc.), but in S class the +TK varies constantly (the highest is +65 on the Ferrari Enzo, but it's not the last car of the S class). I think GameLoft has not yet rationalized the ranks in the S class, except for the F60 maybe? We should wait for the next update.