There have been too many days where I was combat fishing a hatchery hole and switched to a very (too fine usually) leader to see what happens and hooked up first cast. This leads me to believe that line vis can matter.

Some say it is the lighter line that improves action on the offering but in a lot of cases the switch from mono to fcarbon has resulted in the same quick hookup and it was the same diameter.

I usually don't care about line sizes or color but sometimes I do. And switching lines is usually a sign the fishing is slow for everybody.