Well, having nearly been taken out by a car last week I've just bought a hi-viz jacket. It may or may not make drivers spot me any better, but I guess if it does I'll never know as the car will spot me and stop.
Now I drive a car and am pretty good at spotting bikers whether they're wearing hi-viz or not but that's because I ride a bike! Bearing in mind most car drivers are too busy using mobiles etc rather than look for a tiny insignificant biker anything that may help me be seen on my commute can only be a good thing.
I would not make hi-viz compulsory though, but would make driver awareness of motorbikes a compulsory part of car training.