Armor made of mithril in FK already increases the maximum dexterity bonus you can reach while wearing a piece of armor of such metal and has a reduced weight, but it has a special extra that perhaps makes it so special, armor made of mithril reduces by 1 the ranks of armor proficiency needed to wear such armor, unless it is 1 in which that special case would remain the same.
So heavy armor made of mithril would need only two ranks of armor proficiency and medium armor would only need a single rank.
Thanks