Re: MR while wearing non-medable armor.
He will get slightly less mana per second. But nothing big time.
Meditation bonus takes into account your med skill (humans have 20 med from JOAT), AND your int.
Non med-able armour automatically means no meditation bonus (including the bonus from int).
So that means even elves and gargs without JOAT will get slightly less mana per second if wearing a non med suit.
However, don't worry about it unless you are trying to really optimize the suit to squeeze every ounce of performance out.