Depends on the track you're running on. Are you planning on running every month at an HPDE track event? Planning to travel to various tracks?
I've got the stock gearing for a 2010 1SS and it's plenty good for the track near me. If you're going to change the diff gears to 3.91 take note that Chevy ALSO changed the trans gear ratios in the 1LE package to come up with a combo that would balance the 3.91.
If there's too much torque at lower speeds it becomes difficult to smoothly accelerate out of corners and into/through transitions. If the gearing is too tall (for high speeds) then either 2nd is too high or 3rd is too low. If the gearing is too low (for 1/4 mile type speeds) then you spend too much time shifting between 2nd and 3rd trying to find the right combo for corners, and believe it or not that can actually slow down your lap times.
Ideally you want a smooth and usable 2nd gear for low to medium speed corners and 3rd can handle everything else, unless it's a high speed bank or sweeper. You don't want too much power too quick or the back end will want to kick around on you when you're trying to accelerate out of a corner or out/into a transition between corners.
My recommendation before you start messing with the gears in the differential is to spend the money at a reputable high performance driving school and learn how to wring the most out of what you already have. THEN you'll have a much better idea of what mods you want to make because you now have the skill and experience with your car to know what needs improving, how much, and why.
Blue Angel is here!!
1SS/RS LS3 M6 IBM