Invulnerable Coat of Arnd - should be finemail?

[Gothire]

The DMG says that the Invulnerable Coat of Arnd is a set of chainmail +2 (with other properties, obviously). However, as Adventurer's Vault has been released since then and introduced other materials for magic armor to be made out of (finemail, in this case), it seems odds that the Invulnerable Coat of Arnd should give you a lower AC than ordinary +2 chainmail! Also, since the suit can be changed at-will into scale or plate, in theory those should be scaled up in terms of material as well.

What are people's thoughts on this? Should the Invulnerable Coat of Arnd be considered to be finemail or the equivalent for scale and plate? Or should it be treated as-is, since it gives so many other powers?

 

[Pyrex]

Given that it's +2, it probably should be Finemail (or it's equivalent in Scale/Plate).

But its already so overwhelmingly amazing that it really doesn't need to be. In the face of the bonuses it grants the minor AC "penalty" is basically trivial.