Big numbers and whether the ship has viewPortRatio or not don't change weapon fire positions.
Related:
Damaged device overlays don’t take posZ into account.
posZ does nothing when used to define the origin of a shot for a weapon, in the weapon
Odd, I use <DeviceSlot> posZ extensively in TBR and it seems to work fine. Got examples on this one?
EDIT: I stand corrected, I use it in <Device> tags, not <DeviceSlot> tags. My bad.
I believe this is fixed in 1.8 Beta 2. At least, I could not reproduce it. I added posZ="20" to the Sapphire <DeviceSlot> (the generic weapon one) and then created a new game and it worked. I subsequently tried installing a new weapon and the new weapon also appeared at the correct position.
I you can reproduce it in 1.8 Beta 2, please re-open.