Over a long period of time (such that initial 100% is insignificant), with a 1.1 attacks per second crossbow and 12% quiver (1.1*112%=1.232 attacks per second), and 5% crit chance, you are critting 15.51% of the time with sharpshooter.
Add attack speed, such as using a bow, and you get:
1.4*112%= attacks per second (bow) -> 13.50% effective crit chance.
1.6*112%*115% dual wield bonus=2.06 aps (2.032 if additive) -> 12.59% effective crit chance.
Add movement, reducing how often you attack, and you get:
Attacking every 2 seconds -> 22.06% crit chance
Every 4 seconds -> 31.38% crit chance
Add crit chance beyond the base 5% and:
15% crit results in -> 22.50% crit chance
25% crit results in -> 30.44% crit chance
In other words, stacking more crit chance reduces the effectiveness (of the gear, or the passive, whichever way you look at it).
Now, if you shorten the fight to a realistic time and begin with 100% crit chance for the first few shots, you get:
2 minute fight (boss) -> 31.08% crit chance
And that doesn't factor in the fact that you are also moving on most boss fights, which would make it have an even greater benefit.
As you shorten the duration of the fight even more, the benefit increases hugely, such as burning a pack of rare mobs, which is probably when it's needed most.
So TL;DR of the TL;DR:
Worst case scenario, sharpshooter raises critical hit chance by 10.5%.
Shorter fights can make this benefit 25-45%, or in the extreme case, allow you to fire off enough guaranteed crits in a couple of seconds to kill the mob.
Attack speed and Critical hit chance both DECREASE the effectiveness of sharpshooter.
Typical move-shoot-move kiting increases benefit of sharpshooter significantly.