I'm a fan of compression, no question about that. I think it always has to be "tempered" a bit though, and the "big picture" (gear ratio, exhaust pipe configuration, even weight of the kart) can all come into play. When the gear ratio is low, the kart is light, and the exhaust pipe is "mild" (e.g. early direct-drive European type racing), the compression and ignition timing can be amazingly high. But slip a clutch to 10k using a radical exhaust pipe on a 400 pound kart/driver combo, and that same compression will quickly melt parts!
That said, I don't feel 9cc (7cc in the chamber) is too high for most "sprint kart" applications with a mostly stock KT100. Might not be a bad idea to run one heat-range colder plug though.
PM