The upgrade 'Delusions of Granduer' is very similar to the perk 'The hunger' on Voidwalker. If you don't know what the hunger is, it just increases the duration of the energy drain effect. With the upgrade 'Delusions of Granduer' your siphon abiltites are improved.
Almost identical to the hunger. 'Delusions of Granduer' needs to be more useful.
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Are you sure that they don't "combine" to give an even greater effect?