The third roommate has a Breton with his magicka at 3x intelligence. Is this simply because he's concentrated on Magic
A Breton has concentrated Magicka, ie, more Magicka. If you want to go magic, you have to go Breton and take "The Mage" sign. Otherwise you'll NEVER have enough Magicka to cast the top spells (the ones that cost 350+ Magicka).
In Hindsight, the Breton's Magicka bonusses are very powerful yes. My Dunmer gets some decent skill bonusses and a nice power for lower levels (that guardian ghost rapes every monster until I'm level 8 or so), but whether it's enough to offset that extra Magicka, I doubt it...