The widow's base damage is 368. The grenade launcher's base damage is 400.
Assassin cloak adds 75% damage to a shot from your weapon while cloaking.
Assassin expertise (passive) grants you another 15% damage to assassin cloak and 15% damage to the base weapon itself - meaning you get 30% damage when using cloak.
Plus the 50% universal headshot bonus and the 50% universal sniper damage bonus and don't forget the 10% headshot bonus from visor, you get another 110% damage bonus when sniping headshots.
And last but not least, don't forget with tungsten ammo, you get 75% against health and armor.
In other words, at the end game, when the infiltrator cloaks and snipes a headshot with the widow he can get 1100+ to 1300 (against armored enemies) damage per shot.
That is why the infiltrator never uses heavy weapons (well, save for Cain with its 10000 damage).