You're not going to believe this...

[Perfectlap]

sounds to me like your are looking at a minimum of $500 if you only replace one tire and do the alignment(the cheap way out). $800 if you replace both rear tires as you should.
Find a speed shop that works with lots of sports cars for the alignment, I wouldn't give this to the Porsche dealer to do, they will give it to you. Ask them to give you a print out of your alignment numbers before and after the job so you can see how much out of whack your run in with the divider cost you. If they won't go to another shop, its a simple request that a competent shop should be able to do no problem. I slid on some ice once and nailed a high curb with my left front wheel going pretty fast. The tech at Porsche dealer said the alignment wasn't really that off from the other side but an alignment was needed on both.

Look at it this way, you probably needed to an alignment anyway and new tires were on the horizon at some point. Better now that the driving season is closing upon us.

p.s.
tear off the the other mud flap. I ditched mine when I installed GT3MKII style skirting.