I am going to say no on the stock rear gear. The two most critical corners are 2nd gear (current config) - Devils Elbow and the Chicane. Both lead out to big straights, the latter onto the monster straight...you won't run out of gear in 5th with a 4:88 or 5:12 (like we have). You just won't get the jump you need onto those straights with a 4:30 or 4:10.
IMHO, the only time I would run anything below a 5:12 is if I was doing primarily enduros. 4:30 at the low end, 4:88 on the top.
AB