This loophole has been bothering me for a while, and I'm actually hoping someone can debunk it. I've found a way to overcast at twice your magic rating, almost in the same round. Let me explain.
1. I want to cast uber-gigantic Fireball of death, and I have a magic rating of five. I'm a bastard though, so I don't really want to do this myself.
2. I summon a Spirit of Man, overcasting on the conjuring roll ('cause I'm a bastard, remember), to get me a force 10 spirit.
3. As part of summoning spirits, for every 3 points of force, you get to pick an extra power from a list of powers for each type. Charmingly, one of the options for spirits of man is to teach it a spell you know. I teach it Fireball.
4. Now, in the next pass after I summon the spirit, I can give it an order with a simple action. I believe I can then have the spirit overcast based on It's Magic rating, which is 10, since it's a force 10 spirit. Therefor, my uber-gigantic Fireball of death goes off at force 20, and using the Spirit's dice pool of 20 (it has magic 10, and knows sorcery at it's force) to determine successes. I have used this, and one of my players (who figured it out for me, and asked about it before springing it on me in a game) has since used it to avoid clearing a building.
---I want to list drawbacks to this course of action, especially if it becomes a habit.
1. Conjuring the force 10 spirit will be difficult, as you have to roll against it's force of 10 with whatever your conjuring+magic is, and may not work on the first try. Regardless, you're looking at physical drain for that summoning.
2. Should you succeed in getting your spirit (you get 1 service per net success), this action, which the spirit is compelled to do, will probably severely damage the spirit (I've run this a few times myself, the spirit usually lives). At the very least, this spirit will consider itself ill-used, and my badmouth you on the metaplanes. If this is it's only, or last service, if my go out of control on the spot (unsure of rules on this, it actually just occurred to me).
3. The authorities have a giant shopping-plaza-sized lighted sign at the scene with your astral signature scrawled upon it at force 20. This will take 20 hours to dissipate naturally. I reason that it's still your signature, because security mages are supposed to be able to analyze an intruding spirit and track it back to you. Thus, part of your signature is imparted on anything the spirit does with it's services.
and a note to anyone who considers magic unbalancing: In the short term, yes. But you have mages too. And, unless your player has masking, or even if he does, and he's a careless one, Mages are very trackable in astral space. Watcher spirits don't need to sleep, after all.