Like this: http://sites.google.com/site/onthehedge/house-rules
I'm using Arts as Abilities, yes. I'm also allowing ANY companion to be built like a Mythic Companion, i.e. with 20 Virtue points. (I'm keeping the +1 Minor Virtue, raising the maximum total to 21 virtue points, to the Mythic Companions that have the core Virtues, however. Just to encourage taking them.)
All of this has not resulted in overly impressive desire to play Companions, however. At age 37, the age of the magi in my party, I find that the magi are immensely powerful in comparison to the Mythic Companions.
Edit:
Example of my last fight - the party is fighting a massive magical boar. It casts a Me spell on some; the militant Mythic Companion, having no MR, flees in terror. The rest proceed.
The boar has high MR and high Soak, so is very hard for both magi and companions. They attack him physically, but cannot penetrate his tough hide. One magus attacks him with magic, but cannot penetrate his MR. Dead end? Not quite.
The militant magus does his min-maxed shtick, manages a Penetration of 30, and makes the boar very thirsty. The fight is basically over, unless the boar manages to drink soon. [Curse of the Desert]
The other magus expands some raw vis, and again manages to penetrate, with a spontaneous spell that turns the board's legs into flippers. He's like a fish outside water. Hh.
Thus, basically, ends the fight.
Magi are just way, way more powerful than mythic companions. They are more powerful in brute strength within their speciality, have access to resources that make them even stronger (raw vis), and critically they are also immensly flexible (whereas a Mythic Companion has only a few schtiks).