Something critcal to consider in this discussion of "stock" versus "aftermarket" suspension which is all too often totally overlooked in conversations like this one:
Throwing the word "stock" around without saying what year, model, model of the bike, and especially how heavy the rider is (140 lb versus 220 lb) is a big mistake. Not to mention how many miles are on the "stock" suspension in question.
One guy may weigh 150 lb, and have a brand new '08 GSXR, which you can get the right sag numbers on, and set up reasonably well if you know what you're doing.
Properly ajusted, that bike could take you into the Advanced group.
On the other hand is a 210 lb guy with an '03 GSXR with 10,000 miles on the "stock" suspension, which back then was way softer than they are today.
That bike will put its rider on his head long before they get to the front of the Intermediate group.
Both these suspensions are "stock" but they couldn't be more different situations for the two riders.
At Turn One Racing, we set up suspensions all the time, some stock, some aftermarket, and you absolutely must consider these variables when making decisions, especially when your life is on the line.
Yes, I sell aftermarket suspension service and components, but when I work with a customer, and his stock forks have adequate valving, and the sag setting we perform shows the springs are adequate for his weight, I tell them they don't need aftermarket forks. Just refresh them with oil and seals every off-season.
And when a guy comes to me and he needs totally different springs for his weight, the clickers are all the way in and the bike is still bouncing like a basketball, and the forks have 10K miles on them, I tell him not to risk crashing by pushing his personal limits on a racetrack until he resprings and revalves his front forks.