Are the spell-users and non-casters close enough that balancing it could just be a matter of slowing down the number of spells they acquire and maybe their caster level, or is it still that the effects of some of the high level ones are just game breaking still? (Would it fix the problem to make the spacing between new spell levels slow down and just ditch the 9th and maybe 8th levels? Would that strip the casters of too much fun stuff?)
It looks on the surface like the problem of one-and-done for the low level casters have been fixed by PF. Does that seem to hold in your actual games too?
I'll put it this way. Because of the nerfs (and they're
needed nerfs) to polymorph in general and wild shape in particular, a druid who turns into a bear or tiger cannot match the output of a barbarian/alchemist with elemental rage, feral mutagen, and a bunch of other non-core stuff.
However, the benefits of having a spellcaster are still considerable. I believe the power of the non-casting classes has been amped to the point that it takes a few more levels for spellcasters to beat noncasters. But I think noncasters would still have the advantage, as they can put the noncasters into an "out of context" situation.
For instance, one encounter we did in Kingmaker involved a lich-like creature (not the final boss, for those who are familiar with this adventure) whose level was on par with the PCs. He was an 8th-level wizard, but his template pushed him to CR 10. Unlike a lich, he didn't have DR or (presumably) a paralyzing grip.
His position, standing on a ledge about 30 feet up, was almost as good as flying, and he stood back and hit us with a Fireball right at the beginning as we got off a boat. I don't know why it hurt so much... metamagic? luck of the dice? ... but we were worried right from the beginning. After that, there was a struggle to actually get to the wizard. The magus PC used Spider Climb and my druid turned into a Huge bear (15 feet tall with 15 feet reach) and could reach the wizard at a ledge on the top of the cavern. So, a balanced encounter? Probably. It seemed like it was playtested. It was certainly fun for those of us who could contribute.
But that wizard could have aced us, if only he's used Greater Invisibility and prepped more than one direct damage spell. My druid could have located and killed him, but that's because wildshape gives abilities like Scent which aren't available to noncasters. A noncaster, say a barbarian with maxed Perception, couldn't locate him due to the huge benefits to Stealth granted from being invisible, and then would have to use a much weaker attack than the usual Power Attack with greatsword that never misses
Turning invisible is an out of context problem. Unless you've got Invisibility Purge, the whole party cannot take on the wizard, and even then, the purge has a limited AoE. (Or you could use See Invisibility, in which case only one PC can contribute. There are items that let you see invisible creatures, but they're too expensive for 8th-level PCs who can reasonably take on opponents who can cast 4th-level spells.)
Flying is a quasi out-of-context issue. It would hit my group more than most since we're mostly "melee monsters". Even the magus.
In another case, we went through a much-less thought out encounter a bit later in Kingmaker. It was us versus a whole lot of identical barbarians. (I couldn't pick out a leader, a shaman, or what have you.)
We had a new player and he plays a witch. He started the battle with Sleet Storm. I had no idea how good that was, it's only after playing 4e that I could see the benefits of such a spell. It's an AoE effect, a
very large one, that blinds everyone stuck in it, no save. Well, not just blind, they also get total concealment, so the rest of the party needs direct damage, which we didn't have (the magus hadn't learned Fireball yet, and the druid hadn't prepped Flame Strike). In addition, the victims of the spell cannot move unless they make an Acrobatics check DC 10, which is actually not so easy if you didn't put ranks into it. (It's Acrobatics rather than Strength because you're struggling against icy ground rather than wind.) The barbarians probably had Acrobatics scores of +2 (Dex and no training, I figure), so they could make the checks only about 40% of the time. They came out piecemeal...
Right into the Spike Stones (Perception DC
29, 25 + spell level, no that's not balanced for an 8th-level encounter) that my druid had cast, which did damage and slowed them down more.
And then the witch started dropping Web and the magus and my druid started using fire and Call Lightning attacks on the trapped barbarians, one at a time... Due to dumb luck, my druid took a lot of damage from only two thrown spear attacks (but one was a confirmed crit with a x3 weapon). Other than that, we completely owned the encounter. I should point out the magus isn't a "full caster" and my druid took a high Strength instead of Wisdom score since he's a wild shape specialist, so the only difference was having one almost-full caster. Honestly, if we had a wizard, we'd be kicking even more butt due to the wider array of attack and utility spells available.
As Dandu said, some of those spells are poorly written, often with no change from 3.x. There's some assumptions that need fixing (the Perception DC of Spike Stones is simply too high) and then there's some spells that are pathetic (Entangle, I'm looking at you; yes, the AoE is huge, but the effect is low and easily escapable even without a save).
