How does the Weapon Champion's Favored Weapons feature interact with the Daisho Expertise feat?

Question

The Daisho Expertise feat says:

Prerequisites: Proficiency with the katana or wakizashi.

Benefit: Increase the damage die of katanas and wakizashi you wield by one die step. In addition, you may use your Dexterity modifier instead of your Strength modifier when making attack rolls with either weapon. This feat counts as Weapon Finesse for purposes of meeting the prerequisites of other feats.

Special: If you are proficient with the katana as a martial weapon, you meet the prerequisites for this feat, although you only gain its benefit while wielding a katana in two hands (to gain these benefits while wielding the katana in one hand, you must possess Exotic Weapon Proficiency with the weapon). You must possess Exotic Weapon Proficiency with the wakizashi to gain the benefits of this feat with a wakizashi.

And the Weapon Champion Archetype has the Favored Weapons (Ex) feature:

At first level the weapon champion selects a group of weapons (from the fighter’s list of weapon groups for the weapon training ability) to act as her favored weapons. She is proficient with all these weapons. If the weapon champion has a feat that applies to one of these weapons (such as Weapon Focus), it applies to all the weapons in the group.

The wakizashi belongs in the light blade group.

Does the Weapon Champion's 1st-level feature now make all light blades use the Dex stat to hit and treat them as one die higher for damage?

Because as far as I can see, it the wakizashi fulfills 2 criteria, it belongs to the light blades group and is part weapon-specific feat that can apply to the shared weapon group.

Answer 1

Depends on whether you and your DM will see eye-to-eye on how to interpret the phrase "applies to one of these weapons (such as Weapon Focus)". This could mean either "any feat which applies to any weapon from the favored list" (your version) or "any feat that applies to one weapon (like Weapon Focus) whisch is also on favored list".

Daisho Expertise does indeed apply only to several specific weapons - but not in the way the example feat does. Weapon Focus requires you to select one type of weapon, and shares this mechanic with several other feats (for example, Improved Critical or Weapon Specialization). As Weapon Focus is mentioned as example of a "weapon-specific" feat, the intent likely was to allow only this kind of feats to be used with this class feature. Moreover, Daisho Expertise applies not to one, but to two weapon types - that, too, might be seen as not being specific enough. I would say that it does not fulfill the requirement of "applies to one weapon", and thus cannot be used in this way.

P.S. But the main problem this DM would have with this combo - this feat is explicitly a better version of Weapon Finesse, and is only balanced by applying to katana and wakizashi exclusively. When used in the way you have proposed, it circumvents that limitation, which makes it very much overpowered compared to Weapon Finesse.