Myr Superion is a card that makes Johnny players like me want to break it. Gavin Verhey already took a stab at it here as part of a Birthing Pod chain. Sacrificing a one-drop to get a 5/6 does seem like a good deal, but is it still a good deal when you get it on turn four or five? In Legacy, Aether Vial offers the possibility of dropping a Superion on turn three, but again, is that good enough? Most of the decks running Vial are based on tribal synergies (Merfolk, Goblins) and don't have much use for a Myr, no matter how large. The one deck that utilizes Vial and could use a big artifact creature? Ravager Affinity. However, Affinity has no backup plans to cast the Superion if the Vial plan doesn't work out. That probably makes the card too inconsistent.
Sadly, mana "creatures" like the Spirit Guides and the new Chancellor do not power out a Superion. A creature is only a creature when it is on the battlefield. In any zone other than the battlefield, they are creature cards. It's an area of the rules that we often see trip people up, but it would take far too much effort to rebrand everything as creature permanents. Oddball things like a Tezzeret changing an Everflowing Chalice into an artifact creature does work.
By far the best thing about Myr Superion is its German name: Ubermyr. Yeah. Getting a foil one.