Nightvision goggles are equipped in the head slot and currently they are an alternative to equipping an armored helmet. There are also various weapon components (sights, flashlights...) that serve a similar purpose, so it is entirely possible to take a different approach there.
We haven't changed the AP scale/balance recently - the variation you are seeing is normal since AP depend on character stats, character level, morale, tiredness and other status effects.
Enemy sight is somewhat limited when the enemies are unaware (in exploration mode) to facilitate stealth combat. Suspicious enemies, like ones that discover a dead body will have their sight increased. In combat against aware enemies basic enemy sight is no different than mercs' sight, although, once again, it may be improved by e.g. a weapon component.