I'm simply not going down the route of messing about fitting winter tyres. I've been driving in adverse conditions for decades now and I've always used standard tyres and never had a crash - and the only time I ever got stuck was in a Merc and that was because of its nanny traction control. It's maybe different in parts of Europe, where winter tyres are mandatory at certain times of the year but I live in England - the British Isles. Britain's weather is almost unique in that we're in the middle of where five main air masses meet - tropical maritime, tropical continental, polar continental, polar maritime and Arctic maritime. So, depending on which way the wind's blowing - or where the various tropospheric/stratospheric wind currents like the Jet Stream are going - the weather can be freezing cold, snowing, freezing rain, hot, pouring with rain or drought. And nobody can predict the weather here beyond tomorrow. Unlike Europe."A go" meaning what? Until you hear a crunching sound which will cost more to repair than a set of winters? As it occurs to me, the tyres are the only thing between you and the road - summer and winter. I am not disputing driving skills etc. etc, I only see this as a safety feature. You can't control everything, there is always the possiblility of someone driving into you - even more so in the winter with a little grime / snow on the ground. This is something you can control.
I know that various car manufacturers are selling winter tyre exchanges (for a hefty price) which may be sensible in Germany - but this ain't Germany. The point is that you can't even work out when to fit the winter tyres over here. Snow can start in September and it can end in July - or it can never come at all. And the really cold weather can come in March - or even later. In fact I was booked to pick up my 718 from the dealer in early March but the weather was so bad that there was nobody at the dealers - because it had snowed and they couldn't get in. I could have got in, but they couldn't.
Strangely enough what I did with the useless Merc SL I had was change the standard tyres to Vredestein - as you have mentioned - or "Vershteen" as the dealers called them. They were standard summer tyres but they had a good snow rating - and they had a tread designed by Giugiaro (if you can believe that). They looked great. And they made the car driveable without needing winter tyres.
As far as I'm concerned if the tyres don't work all the year round they're no good. Like I said I'll give the P-Zeros a shot at snow and if they don't work I'll take them off, but I won't put on winter tyres. I'll change for tyres that have a more sensible rubber compound.