Well Jmac,

tell me can u avoid PIO???? -- No will be ur answer..

cache hit 99+ would mean that u have all bad sqls running that are eating up ur CPUs...and are subject to serialization
Ur seeing this high cache hit 99+ due ur Buffer gets (LIOs) are so high that PIO is not seen as significant number...

So reduce ur LIOs if u can and never look HIT ratio, its just a crap.

Abhay.