Albert
I think a lot will hang on the New Employment Model (NEM). If we retain age 55 for a full career pension and retirement then you won't have to go past 55 (I think this is unlikely though and the NPA of 60 is going to be set). I suspect it will be go to age 60 (which is likely under NEM) to get a FAFPS pension without doing 20 years and don't forget that you will get an immediate pension if you are unable to continue service under medical grounds with a disability pension regardless of how many years you have served (with the same rights as AFPS05).
If it were me I would serve another 5 years past 55yrs to get a full career pension on FAFPS plus my protected AFPS75/05 pension, rather than leave at 55 and have to wait 17 years for the rest. But that is my opinion.
LJ