Default value after clean GWX install for visual light is 5.0. Also GWX's visual sensors use compartment's crew efficiency, but M.E.P. v6 don't.
GWX default parameters
;Visual.
Visual range factor=0.5 ;[>=0]
Visual fog factor=1.1 ;[>=0]
Visual light factor=5.0 ;[>=0]
Visual waves factor=0.8 ;[>=0]
Visual speed factor=0 ;[>=0]
Visual aspect=0.9 ;[>=0]
Visual enemy speed=0.2 ;[>=0]
Visual noise factor=0 ;[>=0]
Visual sensor height factor=0.4 ;[>=0]
Visual already tracking modifier=600 ;[detection probability modifier], most accurate, once a contact is detected it will lose it very hard
Visual decay time=200 ;[>0] already tracking bonus decay, in seconds
Visual uses crew efficiency=true ;[true or false]
Btw I don't see a point in modifying sensors.dat at all. I think I will use MEP only as graphics mod.
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EDIT: actually there is need in modifying Sensors.dat for your watchman so he will be able to detect ships as far as you (and similarly not able to detect in poor conditions). But for my game I use another sensors.dat from awesome Rocket mod for GWX (version for 16km atmosphere)
http://www.subsim.com/radioroom/showthread.php?t=151612