Powerful Sneak is in the Advanced Player's Guide.
You might want to use Paizo's PRD instead. Fewer errors.
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Powerful Sneak mathematically lowers your damage pretty much any time you ever use it, I don't suggest taking it. Treating 1's as 2's increases the average damage on a d6 from 3.5 to 3.67, an increase of +0.17 damage per sneak attack die. But the -2 to hit means you hit 10% less often and thus ALL your damage (not just the SA dice, but also base weapon, str bonus, enhancement bonus, etc...) decreases by 10%, effectively. Considering 10% of 3.5 is 0.35, it's plainly obvious you're not even getting a 10% increase in sneak attack damage, let alone the other components of damage that are taking the hit.