My 2p's worth of experience running stretched tyres.
The risk for me is not if the tyre comes of the rim, but how safe the car is in the wet (if you don't intend to slide or drift).
If you take a car on track and try reasonably hard, you will find that the edge of your tyre is rounded, as the tyre has distorted under load. This isn't an issue for tyres that are in manufacturers recommended limits, as there is still a treaded section of tyre making contact to the tarmac. In fact after a few laps and tyres have rounded edges the car is much more predictable.
When an over stretched tyre distorts under load, the distortion brings the sidewall into contact with the tarmac, just a little no doubt during fast road use, but enough in my experience to lose traction earlier in the wet than a stock tyre size.
This is my theory based on my experience of seeing stretched tyres sliding a mates car in the wet.