In Pillars 1, playing a caster really did feel like you were playing a badass mage. And while tons of people hated that spells were per-rest, I think per-rest spellcasting was what allowed casters to feel so amazing.
When all your spells are per-encounter, your highest level spells feel underwhelming. There's no reason not to unleash your full arsenal of strongest spells every fight. This makes casting powerful spells feel mundane. Imagine if in pillars 1 you casted minor avatar and storm of holy fire every fight...
The empower system does not nearly come close to emulating this feeling. First of all, you're boosting the spell's EFFECTS and not casting a better spell. Having your numbers be slightly more inflated in a boss fight doesn't feel like anything because bosses have inflated numbers too. In addition to the numbers, it's the spell's effects, the animations, and the consumption of scarce resources (high level spell slots) that makes casting a high level spell feel impactful.
Not only that, but the fact that all spells are per-encounter means that the spells' effectiveness seems to have been drastically lowered (damage and debuff spells are near useless but enchantment buffs are incredibly powerful, but I'm just going to write that off as beta balance. I have faith that this will be balanced by release). High cast times and low reliability on most spells makes sense given that they're per encounter, but why be per-encounter in the first place? Spellcasters are traditionally balanced around having scarce resources but powerful effects for spending those resources. Since spellcasts are per-encounter now, that means they're just mediocre effects for spending common resources.
Obsidian, I'm begging you to reconsider making all spellcasters per-encounter spellcasters. The resource system of caster classes and non-caster classes feel so homogenized right now that I can barely tell the difference between playing a paladin and playing a wizard. At least split the casters into per-encounter casters and per-rest casters to appease both the people who enjoy being able to blow their load every fight as well as the people who enjoy resource management and being able to pop off in boss fights with spells they can't afford to cast in more mundane encounters.