Can you explain how you are using a katana with your Dex-build example? You can't use Weapon Finesse without taking a dip in Swashbuckler for Swashbuckler's Finesse, and the Katana requires Exotic Weapon Proficiency to use one handed (which is necessary for it to be used with Spell Combat). You also can't get Dex to damage on it without the Agile property, because Slashing Grace no longer works with Spell Combat.