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97-00 LS1 = 345 HP (some unmarketed improvements in 99-00 made more power tho).
01-02 LS1 = 350 HP
01 LS6 = 385 HP
02 LS6 = 405 HP
The Auto comes standard with the 2.73:1 rear gear ratio, and the 3.15:1 is optional. All Manuals come with a 3.42 rear gear.
Yes, the 3.15 will make a noticable difference in the Auto over the 2.73:1. Some save money and opt not to get the optional 3.15 when ordering or purchasing, and buy a 3.42 rearend from someone to put in thier Auto.