I don't think that's a fair trade. Do you?
For the tac vest, it's close. That's why I said if it had 5% psi cost reduction I'd be all over it. You also show a lot of numbers but aren't really appearing to consider actual use cases.
Why are we assuming basic 200% bullet resist? If we're not using Kevlar, what are we using? Not black cloth - the armor penalty is so high we're not sneaking in this thing no matter what. Aluminized? Then the heat resist from the cloth stacks with the psi crab, getting us to 70%/25. Since standing in flames is now capped on damage rather than being a flat % of max health, that means you can safely stand in fire and take no damage, which could be a big deal if you like molotovs and also are Tranq specced, or if you use Exothermic Aura defensively a lot and end up standing in the lake of fire you generated even after the ability wears off. Also makes the Industrial Bot fights trivial since their flamethrowers won't much bother you, even without Metathermics in your build. But maybe we're using Kevlar cloth after all. In that case, it's pretty easy to get bullet DT over 120, making you fundamentally immune to pistols and SMGs and most ARs. That means when you're fighting groups of enemies with firearm weapons, you can safely ignore several opponents. That makes some fights a whole lot easier, tactically and mechanically.
And really, who cares about armor penalty? You're not wearing this to sneak, and if you're so fully psi that you're wearing psi tac vests you don't really have much investment in dodge or evasion. So, what? Move points? Like I said, psi haste ignores armor penalty, and moving around in real-time mode doesn't appear to be affected by armor penalty. So you're only slow on the turns when you can afford to be slow. Heck, if you went into it expecting to use psi crab, you maybe even picked up Sprint to go with psi haste, in which case you're highly mobile even with 95% armor penalty.
Even without considering optimized cases, there are further advantages that the psi crab brings. Tac vests are notoriously garbage against melee attacks, but with the extra threshold from crab you can survive stealth ambushes a little better, especially when stacked with the effective threshold of Cryo-Shield (which presumably a cautious psi player keeps up when walking around unknown areas). Electric resist is hard to come by, but crab carapace offers a fairly decent amount of it, and with galvanic vest and galvanic padded boots can get the player up well over 60%/30, which is a fair bit of protection.
Is it worth it to me? No, I don't use it; everything it offers all combined isn't worth just the 10% cost reduction alone to me, because I've played this game so much I only get hit when I want to so armor rating is purely academic. But with 5% reduction, I'd use it, and for people who aren't fully savants about the game, maybe it's worth it even as it is, since it does offer a lot of survivability that beetle doesn't.
