I know this has been done before...but how ridiculous, exactly, can you make a starting magic-user be in respect to throwing high-powered Manaballs?

Let's face it - there's just something satisfying about blowing an entire squadron to smithereens with one well-placed AOE spell.

Well, you can get rather rediculous about this, here's one way, keeping to a 12 for Avaliblity, sense I assume you mean for starting characters.

Aptitude Spellcasting, Magic 6, Spellcasting 7, Mentor Spirit - Any with a Bonus to Combat Spells, Spellcasting Focus 3, and for drain, Make them an Elven Shaman with 8 Charisma and 5 Willpower.

ManaballS. Going for optimisation:

Magic 5, Spellcasting 6 (combat +2), mentor bonus +2. Power focus +2. Only qualified spell defense from a shielding initiate will help at this point, if that isn´t present, you multicast.

Dwarf, drain stat Logic 5, Willpower 6, sustaining focus health 3 (*3): Increase Reflexes/Logic/Willpower.

There's a difference between optimization and maximizing, since SR4 brutally punishes hard maxing.

For hard maxing, Spellcasting (with Aptitude)/Combat: 7/+2, Magic: 6, mentor spirit that gives +2 to combat spells, and a high rated spellcasting focus (normally rating: 3, but I think RC has a quality that lets you get gear at a higher Availability). Top that off with a 7 Edge.

So 20 dice before Edge, and that's assuming the rating: 3 focus.