I wouldn't recommend going any lower than stock unless you put taller tires on, or really need more low end pulling power. You really don't get better acceleration out of lower gearing, because what you may gain from the lower gearing, you lose from having to shift more often. I have a 12/40 setup and a 14/40 with 22" rear tires, and the only time I ever even think about putting the 12 on is when i'm planning on using it for towing, or really slow technical trails where i'm pretty much just crawling over boulders.