For posterity's sake, the tier system discussed is this one. To summarize: tier 1 breaks the campaign every which way, tier 2 breaks the campaign in one way per character, tier 3 is always relevant, tier 4 is relevant at one thing, tier 5 isn't relevant even at their one thing, and tier 6 doesn't even get their own thing.
Cutoff for Tier 1
#Cutoff for Tier 1 TheThe cutoff for Tier 2 is 9th level spells, basically. Duskblades, bards, and initiators can never hope to make it past Tier 3 even if they were all gestalted together. Tier 1 is way beyond their reach.
The generally accepted Tier 1 classes are Wizard, Cleric, Druid, Archivist, Artificer, and Spell-to-power Erudite. This is because they don't just get 9th level spells, they get all the 9th level spells.
Melee supremacy at low levels is... questionable
#Melee supremacy at low levels is... questionable WhatWhat makes melee guys good at low levels? They get better BAB than casters, and better HP. Gestalting together two classes with good BAB and good HP won't make the BAB and HP better than the maximum. And creatures at low levels have so few HP that it's generally trivial to one-shot them for even a non-gestalted character.
The real gap you should be trying to close is doing things outside of combat. Fighter//Barbarian hits like a truck, but what if the problem can't be solved with hitting? The wizard prepares charm person. If the mundanes try to solve this problem (say, by grabbing a skillful class rather than a second fighting one) then their combat power doesn't go up dramatically, and there's no issue with becoming "too good" at combat.
#Casting is good if you build for casting
Casting is good if you build for casting
Let's take the barbarian from before, and slap on 10 levels of Factotum and Ur-priest. His Intelligence and Wisdom are probably rubbish, so he can't benefit from all these new cool abilities. He didn't sink his feats into Font of Inspiration, or Divine Metamagic, so he can't do the coolest stuff these classes do. He can buff himself a little bit, but his caster level sucks so bad that the buffs fall off at the first dispel magic.
So no, Factotum + Spells is not a silver bullet. Let people build what they want.