To the point almost that it detracts from the high viz primary objective.
I think you need to define the primary objective. Mostly it's designed to make you visible in headlights from a long way off. This makes sense if you're working on a railway, as it gives the train driver more time to slow down. The evidence that this actually provides a safety benefit for cyclists is somewhat lacking, and I suspect the optical benefit is negligible if you've already got decent lights.
A tabard that does a momentarily
[1] convincing impression of a police officer (with a view to making motorists suddenly give a fuck about other road users), on the other hand, is hi-vis only because that's what police officers wear - being reflective at a distance isn't part of its principle of operation.
I think the research - such as it is - suggests that it's being taken for a police officer that's important. Passive-aggressive slogans like "THANKS FOR GIVING ME SPACE" aren't likely to work any better than standard optical-visibility hi-vis.
The other problem is that while close passes are scary, most cyclists are hit from the front/side at junctions. Any text on your back isn't going to help there. If you actually want a safety benefit you'd be better off trying to cultivate the attention-increasing WTF-factor with a gorilla suit or recumbent bicycle or something.
[1] To avoid falling foul of the law against impersonating a police officer.