I would personally argue that the Mithril Body Feat should give the same PRR/MRR as medium armour,
the armour it grants is the equivalent to a Mithril Breastplate if you compare the two:
Mithril Breastplate
+5 AC
-1 AC penalty to Skills
15% Arcane Spell Failure
+5 Max Dex Bonus
Mithril Body
+5 AC
-1 AC penalty to Skills
15% Arcane Spell Failure
+5 Max Dex Bonus
Both of which are treated as Light Armour, but should provide the PRR benefits of Medium Armour. Especially since Warforged and Bladeforged need to sacrifice a feat to get it, regardless of Class, unlike any other Race which gets Light/Medium armour for free.
The Composite Plating for All Warforged/Bladeforged is also a type of armour, and could possibly also grant some PRR/MRR even if they don't take the feats, to make up for the fact they don't get the benefits of the Class Feats that grant armour proficiency.