+1
Edit: In response to what @Neleothesze said about Sleep. It is important to take that into account, although increasing an enemy's level and simply increasing their HP has the same effect in this situation. But yeah, for weak enemies without levels (e.g., goblins), simply increasing their HP has cascading effects with other in-game abilities. The same is true for increasing enemy ability scores - Oh look, now all enemies effectively get a +1 or +2 to their STs, further weakening ST spells in BG3.
Yes, exactly. For bosses and important quest fights, level and stat/HP increases might act similarly but for your run of the mill encounters, you'd get to use a wider range of spells that would otherwise become subobtimal.
I haven't relied on saving throw spells since i inspected half the npcs/monsters around and found out that they bought their stats Diablo Immortal-style. All that gold we're not getting? It's cause the monsters used it to PTW apparently.