(Relocated to The Arsenal because it probably belongs here)
Subject basically says it all. If I remember, Acheron's brokenness makes it worth upgrading since it becomes much stronger at a more mid level, and some items like Alchemers are nice to upgrade because of their charge. Could I get a sort of list of items to be believed to be entirely worth upgrading to 5* while lowish on resources?
What items (particularly weapons) are worth upgrading to 5*
All bombs are worth upgrading to 5* especially haze-style bombs.
IDK about Irontech though, some people say the 4* is actually better but I never used it so I can't confirm.
Alchemers maybe the weapons that are worth upgrading the most.
Divine Avenger is only worth upgrading if you spam charges (especially with chaos set) and/or just wanna look cool.
Gran Faust too is worth upgrading due to stronger curse and if you use curse resisting gear you can spam charges without worries, also upgrading it gives you a way bigger projectile on the charge.
Gran Faust gets less charge-spammable than Faust. Firstly, the curse gets so strong it's impossible to be immune to self-curse (although you can guarantee only one slot gets cursed so you always have one weapon available). Secondly, the charge time increases dramatically.
I didn't say you'll be immune but only 1 slot will be cursed and for the long charge time i used to use Divine Veil + Chaos Cloak and Purify pet perk so it becomes a bit more spammable and safer.
Did you ever try GF charge-spamming in C42?
Try it it's so fun.
Has anybody tried a high curse resistance loadout with dual Gran Fausts, so there's always one Gran Faust that is not cursed? I kind of want to try that now...
Maybe heavenly iron set (cause med dmg bonus for swords) or maybe divine veil and heavenly iron helm (to counter shock debuff) also consider the set i mentioned above.
Get an elite sword focus module (or 2 if you don't value shock resist or health trinkets) or an elite slash module or 2 if you got CTR UV's on your GF's.
HF with GF charge-spamming ^_^
As you impliled, whether something is "worth upgrading" depends on your resources. Do you mean that you have excess 5-star Orbs of Alchemy but not enough 5-star Fire Crystals?
Upgrade armor and shields always. They don't benefit much from heating, but they tend to have big bonuses attached to the 5-star upgrade: status immunity on shields, and big offensive bonuses on armor and Barbarous Thorn Shield.
As a rule of thumb, upgrade weapons only once you have enough 5-star Fire Crystals to heat them to heat level 8. For that is roughly when they overpower their fully heated 4-star versions. But some weapons benefit strongly from the 5-star upgrade, either by adding features or by changing damage type. I guess that's what you're really asking? I'd say:
* Divine Avenger
* alchemers
* Iron Slug
* maybe haze-style bombs, for the increased radius?