Be glad if they can replace only one disc, because these things are often replaced in pairs. I got the same diagnosis because of small chip in the edge of the disc, single disc costing 9300Eur. Second time they have to replace a disc in my car in the past 2 years, and it's not like I'm off...
Yup, you will fail the check for Porsche Approved extended warranty if you don't have NF0 certified tires. A service guy at Porsche service told me this is the most common reason to fail the check.