Because the 6L80e trans has such a low first gear. Every GXP, Chevy SS or fifth Gen Camaro with the 6 speed auto has a 3.27 rear ratio. Manual trans always need more gear. The GXP and Chevy SS manuals got 3.70 and Camaros got 3.45. The only reason I could see going to 3.45 would be of you were Autocrossing and need more acceleration from corners. You won’t regret a 3.27, you might regret the 3.45.
Only if you have traction. Unless you have much wider rear tires to handle the extra torque. 3.45’s would just make you good for burnout contests. Just like too high a stall converter, too much gear won’t always make you faster. You have to plan out drivetrain. The 3.27 will raise highway rpm about 230 rpm. The 3.45 about another 110 rpm on top of that
Also depends what size wheel you are running. Stock 18” I would go 3.27. If running 20” wheels the 3.45 will be just fine. Mine hooks up just fine even with pizza cutters and extra 100 HP at the wheels.