To which the answer is yes (trade in half HP for superior stats). The BST change from Zygarde-50% to Zygarde-C is drastic due to the immense increase in HP, with some slight differences in Special Attack and Speed. The sole reason that Zygarde-C has a significant BST change from its base form should warrant for it to be tiered differently.
This isn't quite what I was getting at when I meant opportunity cost. Though the loss of 50% HP is an obvious cost of the form change, it's not something we can really tier for since what would we do- allow Zygarde unless its HP drops below 50%? I'd also argue it's not really an opportunity cost, but I don't really care to pursue that since it just leads to irrelevant semantics surrounding definitions. Anyway, what I really meant was that when teambuilding the player decides that they're going to forgo Aura Break in favour of Power Construct. Aura Break has real benefits to Zygarde, whereas Power Construct does literally nothing besides offering the ability to transform into Zyg-C (I know Aura Break is actually really shitty, but it does
something for regular Zyg, where Power Construct does nothing beyond the form change).
Fwiw I've completely lost the train of thought I had when I made that post. Looking back now, I kinda think my opportunity cost point might be a bit of a red herring. In any case, if we were to tier Zyg-C, Darm-Z and the like separately from their base forms, we need to settle on how we feel about tiering non-pokemon, since that's what something like this amounts to in practice. Based on the policy surrounding bans I think it could go either way, since we do allow for non-pokemon bans, but on the other hand they're treated differently from regular bans, so I think that extending this to general tiering could justify either position.
Personally I'm against tiering abilities/items etc, and think we ought to stick to pokemon as much as possible. The reasons I hold this position are for the sake of consistency and also due to collateral, since generally non-pokemon bans have a much broader impact and are just a really crude method of altering a metagame imo. The latter argument doesn't quite hold true here though, since I think in every case where there's a form altering ability/item, it's specific to that one pokemon... I guess the one exception is Meloetta-P, since Smeargle can learn Relic Song.