All exotics are upgradable through Xur. Where is the confusion? He's eventually going to have the upgrade available and you'll be able to take it to 350 or 353.
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Not necessarily. Because whenever you get an exotic now it's the upgraded version (apart from the exotic chest in VoG) so how come you don't get an upgraded vex from Atheon?
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Athion drops 300 exotics. Got the last word from him last week.