Eh... Celerity? Arcane Fusion/Spellsurge?
Yea, most of the archtypically broken spells (Gate, polymorph, etc) are core, but a (Core Spells) Wiz 20 vs (Core + Splatbook Spells) Wiz 20 isn't even a contest. The splatbook version is going to go first and go twice before the core version can act.
Fundamentally, power in D&D is about two things: action economy and versatility. Anything that enhances those aspects of a character increases the power of every asset that character has. "Core is balanced, splatbooks are broken" isn't as inaccurate as you make it out to be when considered in those terms.
Yes, a wizard that uses non-core plus core materials is more powerful than one who uses only core materials. That still doesn't mean that core is more balanced than core+. Try comparing a wizard that doesn't use
any spells out of core with one that uses
only spells from core. I think the core only will likely be
stronger. Maybe not in a duel, due to Celerity, but in pretty much all other ways. The non-core guy can't cast Time Stop or Contingency. So the core guy's contingency trigger can totally be "when X casts Celerity", and then he'll probably win in a duel, too.
The increase in the power of casters when you add splats is minuscule compared to the power increase that non-casters get. That's why core only games are
more imbalanced that core+ games. At least a non-core melee guy can actually contribute meaningfully past level 5 or so if he is an uber-charger (or other optimized build). The core melee guys can't even do that when compared to what an equal-level core caster could bring to the same party. They just fail and fail some more, and then fail even harder.