I think seeing with steel and iron lines may have had to do with an inquisitors inhuman physiology , as they changed, allowing them to see more minute detail in iron lines, but not necessarily act upon them, also there is the kind of change in hearing of a blind person, it grows no more acute, they just have learned to interpret the input. Basically they have no real sight to distract them from iron lines.
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But the greater the sensitivity, there comes the disadvantage of over stimulation, sort of like a greater amount of light can blind one with normal eyes, as in when Vin throws pewter dust at an inquisitor.