After 14,000 miles I changed from PZero to PS4's - they were down to the 2.5-3mm level. I then sold it - Oh well. I did think wow the PS4's are way better - (of course they were newer and the placebo/internet effect!).I'm also in the South, in Wiltshire and agree, 2.5mm to 3mm has to be the limit.
I'm not overly impressed with the P Zeros.
My new car has P4Z's. Super happy, I feel as confident in the twisties as I did with the PS4's. And it is winter too!