On a PA28, you could probably get away with 1mm, maybe 2mm or even more
Can you explain how you have come to this conclusion? I assume you have test flown a PA28 with more than 2mm of ice adhering to the airframe.
It does increase friction but it won't increase the stall speed
Wrong - ice always increases the stall speed by reducing Cl and increasing weight.
Friction? Ice increases friction- so your car stops more quickly on ice does it?
Many years ago the CFI of the Edinburgh flying school was seen to be religiously sweeping all the snow from the wings of a C152. Minutes later he spun in on take off.