Question on Mythic Armor Mastery and Mithral armor

[Tangent101]

I was wondering something about the Armor Mastery Path Ability (for Champions) - it states that with two selections of it, a spellcasting warrior doesn't suffer from spell failure for Medium armor.

Does that include Heavy Mithral Armor? The rules for Mithral state: Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.

As such I allow a character in Full Mithral Plate to sleep in the armor while using the Endurance Feat. But does this also work for the Armor Mastery ability? (It seems like it should, but Mythic is different, so it might be considered semantics.)

[StabbittyDoom]

It's just as the rule states: For absolutely ALL purposes besides proficiency, mithril fullplate is medium armor. There are no other exceptions, nor do I anticipate that there ever will be.

[Tangent101]

Thanks :)