We had this discussion years back when Easyjet brought the A319, with the extra seating.
The B737 operates with minimum 3 cabin crew due 149 pax seats, A319 operates with minimum 4 cabin crew due 156 seats.
Discussion was only 7 more pax for one extra crew member, told that no problem you can still operate with 3 crew member by keeping loads below 151.
This was a no no because it is seats fitted not bums on seats that counts, only relaying which was posted a few years back.