First things first - this is the spell penetration formula, as I know it. If the sum of (d20 + CasterLevel + SpellPen_Feats + SpellPen_Enhancements + SpellPen_Item) is greater or equal to the mobs Spell Resistance, the spell pierces (this of course doesn’t take into account the possible save a mob may make if the spell has a save component). And let’s also agree that end-game mobs have significant Spell Resistance we must pierce to have our spells stick (even pure offensive casters speced specifically to pierce SR have significant issues piercing SR at end-game).
Regarding Feats - In comparing a ‘caster’ FvSoul (who normally stay pure, for obvious reasons) to a melee-oriented FvSoul, it seems to me that melee-oriented FvSouls are somewhat feat starved. We need invest in an entire 3-feat chain (either TWF or THF) and at least power attack (I see these 4 as an ABSOLUTE minimum). Some will fit in ImprovedCrit so as not to be force into MinII. Regarding MetaMagic feats, I just don’t see building a FvSoul without Quicken, Maximize, & Empower. And how does a melee FvSoul live without extend for the short buffs? And Toughness comes with at least 60HPs (feat + FvSoul enhancements), which is hard to pass up. I’ve seen melee-oriented pure FvSoul builds, but I’ve yet to see one without gaping holes in the feat department. This is IMHO, and your mileage may vary.
Point being, monk and fighter splashes bring needed feat slots to properly flesh out the duality to adding melee capabilities to a caster class. This comes at the cost of spell penetration (1 per splash level). These feats are mostly used for melee feats, or toughness, or metamagics listed above, and NOT on spell penetration feats.
So, given a two level splash and no investment in spell penetration feats (which seems the norm for most melee-oriented FvSoul builds), that’s 6 less spell pen than a ‘caster’ FvSoul, who is already having problems piercing SR in elite/epic end-game content. For example, if the caster FvSoul is getting resisted half the time (piercing on rolls 11-20, missing on 1-10) which is much better than can often be expected at end-game elite/epic, this build will be piercing SR much less (17-20, missing on 1-16). That’s a shift from hitting 10 in 20, to 4 in 20 (%50 to %20). And pure casters passing spell pen at even %50 of casts on end-game elite/epic is probably extremely conservation in this example. In reality, the different is probably much worse (%35 for the pure caster, %5 for the above build).
Here’s a list of the five tier-9 spells, and my thoughts:
Energy Drain -- Must pass the spell penetration check. The pure caster with 6 higher spell penetration already has trouble landing in end-game elite/epic.
Implosion -- Must pass a spell penetration check and Fort save. The pure caster with 6 higher spell penetration already has trouble landing in end-game elite/epic.
Mass Heal -- Slow cast time (almost too slow for mid-fight), and no potency 9; situational, at best
Summon IX -- Hezrou; Situationally handy
True Res -- A more expensive resurrection with, rather than half HPs, heals to full. Meh...
Evidently, I just don’t "get-it" with the NEED to have the 9th tier spell slot. It’s either a spell that will hardly land in end-game elite/epic content (for a melee-oriented build), a summon that’s hard to control & dies rather quickly (while gaining agro you didn't want), a mass heal with a LONG timer & without potency_9, or a resurrection that heals a little more than the equivilant tier 7 resurrection (at the cost of more mana).
If a build splashes and/or doesn't get SpellPen feats, wheres the motivation to NOT splashing down to 17 levels in FvSoul, ditching the entire 9th tier of spells, and gaining two levels of one class and one of another (17/2/1)? To save a spell slot for one of these "gems"?
Enlighten me, please...