[Perfectlap]

just my experience, when the tires start to go they go quickly. I check my tire pressure with each gas tank 32F and 34R. (It was the other way around with my old car) and usually do custom alignments where negative rear camber is maxed out. But with this Boxster I decided to stick with Factory specs. I figure Porsche would know better than me.
Accelerating hard/often from a stop (red lights) will significantly eat into your rear tread life.
Your car sounds like it was tweaked often, did the previous owner go beyond Factory specs?
From reading the Porsche reviews on TireRack.com 24k sound like you got your miles worth. Some reviews complained about only getting 15K out of PS's.