I don't understand how a 220 wear rating has less grip than a 300 rating. The wear rating is just like how soft a compound it is. The lower the number the softer the compound.
I personally am swapping out the stock worn rears for the PSS and will do the same with the fronts when they are worn. I don't track the car or I would get a much softer compound.
2011 M3 E93 DCT
2012 X5 50