Tony, I can't testify to hi-vis lines spooking fish in clear water, since I don't fish waters I consider as "clear".
However, I can testify to using them successfully in stained to murky water. And by "stained" I mean water that you can see a chartreuse jig down to a couple of feet deep. I've also fished hi-vis lines next to rods with clear/blu mono ... and had equal number of bites.

I think the main reason for hi-vis line, and certainly for me & my preferred method of casting and swimming a jig over cover, is that you can "see" the line move on the lightest of bites. It's also been said that it helps longline trollers keep track of their lines (esp in turns, or when reeling in fish). Methods that employ floats, or more vertical/stationary line position (like vertical jigging, tightline fishing, and the variations on "spider rigging") .... where the bite is registered as a disappearing float, or a bending rod tip, may not be enhanced by the use of hi-vis lines.

I'm not of the belief that fish are "spooked" by line color. More than likely they are spooked by other circumstances, like the boat, shadows, movement, noise, etc.
Remember .... our predecessors caught fish using black nylon fishing line, long before "monofilament" line was ever invented. (I started my fishing with it) And I can guarantee you that black nylon fishing line is way more visible than hi-vis lines, when viewed from below & contrasted against the sky.

... cp