Unless they've changed the ruling, an adept's Sense Enhancement power is automatically rated at max possible.
The cybernetic Select Sound Filter (in SR 3) was rated 1 - 5. The adept power is automatically rated at 5.
Vision Enhancement, in my opinion, is no different and if the adept selects it, automatically gets it at the max possible rating, in this case 3.
So the adept can spend .5 and get +3 dice to Visual and +3 audio checks (.25 a piece for the Sense Enhancements) or +2 dice to all Perception checks (.25 a piece for Enhanced Perception).
-Siege
Edit: Same thing with Vision Mag - max possible rating.