Everything in DF that costs FPS, and that you can control, involves items and units in your embark zone and the available pathways connecting those items and units. It is not just that your dwarf population rose to 80, but that your item count increased, and that the three-wide hallway is often cluttered with dwarfs that need many small repaths as they dodge past others in that hallway.
Review your heaviest traffic pathways and spend some time watching the units move about. If you see a crowded hallway or room and units are constantly side-stepping past other units as their paths cross, then it would help to redesign those areas. As an example, meeting areas such as taverns and temples should have a single entrance into the room, so there is no available path through that room leading somewhere else, and so there will be no haulers dodging through a crowd of drunks.
The caverns should have only one or two access points to your fort and they should be constricted by raisable bridges that should be raised most of the time. This will limit the pathing calculations that nuisance invaders cost from their attempts to path to the things in your fort that interest them (booze, dwarfs, wood furniture, etc). Do not use locked doors to control cavern access, as building destroyers will still consider them to be something they can path through).
So, if you can limit the items and non-dwarf units and the pathways to some degree, you can reduce the overall FPS costs while maintaining the dwarf population count.
I know this advice is a bit vague, but every fort is its own mutilated bonsai shrub and needs the items/units/pathways concept applied differently.