All FMCs (even silly ones like A320s) are mere computer tools of trade. Optimum level is merely advice to be taken into account with real world realitys (like your FL 250 for extra time to get stuff done). From what you say FL 270 sounds like a short-sector canned level for averaged wieghts and seasonal winds.
I just done an A320 conversion in Tooloose. When the bloke wasnt looking I played around with the box including what your asking here. After you stick in all the winds for varius levels, cost index etc go to the Progress page and stick in varius cruising levels (incl the optimum) and read the flight-time and arrival fuel of each level on the flight plan page. On some sectors I done in the sim, even a level slightley under rec max level saved 100kg over optimum.