I got about 4K miles out of the original Kenda and same from a replacement Pirelli, cord showing on both at replacement. I'm running the Shinko rear and front now and quite pleased with the result. Seems to me the profile of the Shinko is also much rounder, maybe a bit taller, than the previous tires, which appears to allow for a slight increase in lean angle before stuff scrapes.
Good to hear you got 8K out of the Shinko, I'd be happy with that. First I've heard of an Xciting 500 approaching anything like 18,500 miles on a rear tire. Most other reports I've seen are between 4-6K.