This is a topic that's been discussed time and again. Pretty sure it was discussed somewhere that 'named' items weren't meant to be modified and were intended to be unique items that are balanced.
The reason this could create problems with the Mithral/celestial armors is in the reduction in armor types. Celestial armor is considered light armor. Making it out of Mithral now treats it as no armor at all but requires light armor proficiency. Now a monk could take light armor proficiency and wear Mithral celestial armor while enjoying everything about being a monk which is also a cheaper and better alternative to getting bracers AC 8. The Sohei gets every benefit of his class and archetype instead of having to make a choice of armored or not.
Stats for Mithral celestial armor are AC 9, max dex 10, armor check 0, arcane failure 5%(0% because it's treated as no armor)
With the celestial plate armor made of Mithral it's now considered a light armor, but requires medium proficiency. This suit allows for class abilities like evasion and improved evasion or anything else restricted to light armor.
Stats for Mithral celestial plate armor are AC 12, max dex of 8, armor check of 0, arcane failure of 10%.
These two suits of armor would be the only pieces of armor sought after by adventurers as they are now the best options and open up a lot of added viability. Although I'm all for Mithral options of celestial armors just realize it is a very potent adjustment.
Other than the points above fighters are also trivialized somewhat by this option. A fighter with armor training could improve these armors close to this level or even a suit of Mithral full plate to max dex 7, armor check 0. These armors make armor training useless.