I can definitely see reducing it to medium armor like 3e does. Talk to your Narrator!In A5E Mithril (I will never spell it with an a) Plate is still heavy armour and therefore you do not add a Dex modifier to AC. A5E is still 5E and that is how it is was done in the 2014 edition. In 3E/PF1 mithril armour reduced the armour type one category so that heavy was treated as medium and medium treated as light. I have played enough editions that all the rules tend to bleed together in my mind. I am forever looking rules to keep it straight.