Here is some info from a while back.
http://986forum.com/forums/showthread.php?t=11831&highlight=Sumitomo
20,000 miles is good. I see people who only get 8,000 miles.
My Sumitomo HTR ZII has a treadwear rating of 220. Not in the market for tires so I don't know if ZII is still on the market as someone mentioned a ZIII tire.