[Taqwus]

You get higher penetration, less fatigue (marginally; since the formula is ((base fatigue / (extra levels + 1)) + spell-casting encumberance) really high levels make less and less difference), and some spells increase in area or number of effects e.g. Orb Lightning.

No ritual or battle spell requires more than level 9 in anything. Quite a few paths don't have level-9 requirements at all, if memory serves; there aren't that many except for Utterdark, Wish, Astral Nexus, Strands of Arcane Power.

If you're empowering that much, you're probably wasting gems that could more effectively be put into creating and unleashing pet Doom Horror commanders, because the empowerment costs just get extreme, or at a minimum in forging lots of tasty artifacts.