Sharpshooter does give you +10 damage, but it also gives you a hefty to hit penalty of -5 to go with it, making it's use situational, and 16 dex is not really a high requirement since any character regularly using bows(and who else would take this feat?) is pretty much guarenteed to have 16 dex right out of the gate at level one, and even if they dont will be spending their first attribute score improvement on getting there. Also consider all the other feats that give a + to hit as well as features like archery fighting style. You already made a character that starts with a whopping +9 to hit at level 1, you could get +10 at level 3 with this, +11 at level 4, which bumps up to +13 by level 5, actually +14 if you also worked up the weapon feat tree which is double the +7 most other characters will have. Consider also that at level 5 most martial characters will get a second attack, and if both of them hit (which with a +14 is definitely more like than not) they will also be doing 2x +4 dmg. Thats 8 extra damage, in one turn, which given the die of a longbow is a d8 that is statistically better than an attack(maybe even a critical) which, if you're a fighter you'll get another one at level 11( aswell as yet another bonus to hit and damage from this feat!) by the time you reach level 19 you might have an attack bonus of +20 and a damge bonus of +8 without even touching a magical weapon .
