OP is pretty damn correct actually. Energy gating is a real thing, people. I'd rather have Zed have an easier time in lane than have him bounce around with no energy limitations in teamfights later on.
Think of URF. Zed is oppressive as all fuck on that game mode, not only because 80% cdr was the thing but because he actually couldn't run out of resources mid teamfight, meaning he could spam all his skills on CD.
Apply this to a Zed with mana in a regular game. 40% cdr (FH, lucidity, 5% mastery or whatever combination you want) and a large enough mana pool to go through an entire teamfight without running out of mana thanks to FH. As far as skirmishes/teamfights/objective control goes, this Zed is only 40% cdr below his URF counterpart.
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What item build are you refering to and how would this item build be better if he had mana compared to if he had energy? There are only 2 items which are stronger on mana champions and those are the archangel staff and manamune. All other mana items are just as strong on manaless champions as they are on mana champions.
A FH could potentially make him a pretty overwhelming bruiser.