I just got Nirwen's Mercy off of an Iron Banner game. It came with Outlaw and Glass Half Full. And depending on the enemy, it does 460 to the body, and 921 to the head. With Glass Half Full active, it does 466 to the body, and 932 to the head. That's a total increase of 6 to the body, and 11 to the head. That's an increase of 1.3% to the body and 1.19% to the head. Obviously, the numbers will be different for Majors/Ultras and different enemy types. My conclusion is that the benefit of Glass Half Full is so negligable that it is never warranted taking. There is no benefit to using this perk in ANY situation. The perk needs some attention to make it more valuable.