I gave my Chao - Mystic - loads of red Chao Drives and his power level is now 99, but when I give him another red Chao Drive nothing happens. Is it possibly the highest that a level can get up to?
99 is the level limit. However, even after hitting level 99, you can still use drives to change the chao's appearance (unless at that point it's too old for second evolution). If your chao reincarnates, though, it will keep 10% of its stats, so when it reaches level 99 again, it will still be stronger than the last time.

~ Crazy C Lea