Thanks for the answers. The next might be a noob question, but what about metamagic feats? Do they work for Sr lowering effects? As those are a fixed number rather than a dice roll, empowered Nature's Balance or empowered Bestow Curse is probably more efficient than the normal version + Assay Resistance.
The SR-drop values do not get boosted by Empower Spell unfortunately
I noticed I went a lil crazy in the detailed examples posts so I just updated two new in-game logs images on the first post. And a couple here.
Incidentally, they show how effective is casting on the defensive
Some less druid-centric example, let's see how a cleric can stack the deck.
The Gelugon has a base SR of 84.
As you can see (red highlight) the Bestow Curse (BC) spell fails the check, but that only prevents part of its effects:
Bestow Curse lowers all of the target creature's ability scores dependent upon the caster level: [...] Level 40 or more: -12 Also subtracts 1 from the target's attack bonus per 4 legendary caster levels. In addition, the target's spell resistance is lowered by 1 per 5 caster levels for 10 rounds, regardless of spell resistance or save success. [...]
That means that, even if the stat-draining didn't happen due to spell resistance, the next spell, Energy Drain, matched the lowered SR of (84 - (60/5)) 72
Being pure helps your SR-dropping ability so for most pratical situations, a splashed cleric/wizard/bard (39/1 sort of builds) not only loses one spell penetration point due to the CL loss, but its BC is weaker (11 SR drop instead of 12).
Here I wanted to keep the screenshot readable so didn't use prayer/battletide to lower enemy saves, nonetheless the green highlights show you a nice side effect of the drained levels, lowering Gelugon's base fortitude of 49 to 32 in the successive spell; nerfed SR and nerfed saves lead to a pretty clean kill
It's a bit beyond the scope of this guide, and generally speaking people have less issues dealing with high saves than with high spell resistance, but it boils down to not be afraid of 'losing' one of two actions to make the enemies very vulnerable to the successive spells you're going to cast. Especially true when you are going to use high level spells that you don't want to fail, and the price for a guaranteed success would be using some very low spell you have abundance of.
Final funny pic, let's see how badly you can cheat enemy SR
Also, does the Visor of Cursed Chaos mentioned above empower all effects of Bestow Curse to AoE or just the SR drop? If so, I guess it's also very nice to have on a PM or Pariah.
People already answered you so I reply late, but since I was talking about BC let's say you're spot on about that item.
I value AoE BC on Pariah/PM as mandatory as soon as you have the tag for it. It's better than a Asmo helmet even if that would net you a higher casting stat. It's better than the Vestige helmet even if you lose slots. It's better than an Ego helmet even if it isn't of a shiny purple.
That's because with those two quasiclasses, who are overall limited and weaker than standard arcanes, the increased BC efficiency effectively translates in a huge debuff to enemy mobs, and you turn into a very efficient support class, when you don't use that to simply net you more kills. A single casting of it is very slot-efficient as well.
The SR drop is always nice, and with PM boosted caster level you're dropping it by 13 points, 14 if you drained a couple of levels first (and why shouldn't you? ); soul gems are also worth to use just for that, in between spawns.
Usually your SP against most fodder (and ability-drain vulnerable) creatures is so high that the first casting pierces SR already so you don't need to recast it.
-12 to all stats basically means: -6 ab (from STR drop, and goes up to -11 with BC other side effect), -6 ac (DEX drop, usually enemies wear no physical armor to limit it), -6 to saves and skills, lowered constitution.
If that doesn't sound hot already, let's look at it from the player's perspective: it's like giving all your party a stacking +11 ac, +6 ab, +6 dc on all future spells and disarm attempts, and (6 CON modifier x 40 monster level, toolset limitation) effectively dealing 240 aoe damage (that doesn't show in logs )
From a selfish point of view, those boni and especially the effective DC boost after lowered saves, are better than any other item you could wear.