Conceptually, I'm all for trying to address the special material pricing, which makes no sense.
Consider a Large mithral breastplate.
With the new ruling, it's worth (150+4000)*2 = 8300gp
With the old ruling, it was 150*2+4000 = 4300gp
If you melt it down to a huge pile of mithral, it's worth 30*2*500= 30000gp
No matter what, something is wrong here. I would be fine with it being worth less when melt down, but not (way) more... Especially given the rules are very affirmative on the fact that it is no plating.
So, this FAQ solves nothing, and has a bunch of issues, most notably the fact that it invalidates many, many printed material.
Honestly, this needs a PF 2.0, you can't reliably address it with FAQ, unless you are prepared to errata almost everything.