It's an amazing roll. The best possible roll for single stat I've seen on an Exotic chest is 181 for my Lucky Raspberry. 179 (if you upgraded a previously owned piece) is the highest possible roll you can get from an upgraded piece as far as I know. Even still the 2 point difference works out to like a 0.7% increase in CDR, so not worth it to seek other ones.
TL;DR - Don't buy the chest engrame from Xur if you're solely looking for a better roll on your Armamentarium, you won't find it lol.
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Thanks for the info :)
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Just in case you wanted to know I define the following threshold as "good": Helmet: 130+ Arms: 115+ Chest: 165+ Boots: 150+ The above considers Legendary and Exotic single stat. Split Exotics (ie, my Obsidian Mind with 75/70) will have a naturally higher roll than others.