I have some houserules to balance Quickening, which I think you could consider. They keep it powerful, and useful without it being just the absolute monster and arms race it can be.
1) When a spell is quickened, its limit becomes the limit of the spell as cast at its force; modifiers like using edge to Push the Limit or reagents disappear, and any hits above the Limit of the spell are lost as normal.
2) You cannot quicken a spell that has been overcast at a Force above your MAG rating; if your Magic attribute drops below the spell's Force at any point, the quickened spell's Force is permanently lowered to your current MAG and cannot be increased. (You may, however, cancel the spell if you wish and re-quicken it, paying for it with Karma again)
3) The Karma cost to Quicken a spell is (Force) Karma. This is how it was in the previous edition, by the way.
This makes Quickening still a powerful tool, but the investment per spell is increased, making it harder to quicken a dozen max-Force spells on you. It also limits the power of the spells, so you can't simply cast a F12 spell with Edge, get 18 hits, and then quicken that.
A reasonable in-game justification is that for 1), those temporary boosts fade as the quickened spell normalizes and forms to your aura. For 2), your aura cannot properly quicken a spell that your own MAG rating can't normally power; it's just too powerful of a spell for you to sustain this way. Number 3 isn't something that can be keened in-game, and so doesn't need an explanation beyond "it's for game balance".