Despite hi-viz becoming more common, I don't often have to stand out from a fluorescent green billboard behind me.
If a driver doesn't look, they wont see. Hi-viz will not alert them to your presence. Yeah, sounds alright.
If a driver does look, no matter how carefully, it will be easier for them to spot me in hi-viz (compared to them looking equally carefully but with me in dark clothes). When they "look" they are looking for, say, things moving towards them, to make a decision about whether to pull out. If that thing stands out from the background then they will be more likely to pay more attention to it, surely.
I think a study is required though

put drivers in a "hazard perception" style test. Perhaps we could have pictures of a driver's view in various situation flashed up for varying (short) periods of time and see how well a competent driver can identify hazards in what time. Part of this could include cyclists in hi-viz etc. This would simulate the grey area of "looking but not properly". I think we agree if a driver doesn't look at all, we're screwed regardless of clothing; if a driver looks truly 'properly' we can be camouflaged against our surroundings and they should still see us