What do you think about allowing magicians with the Quickening Metamagic to reduce the effective Force of Quickened Spells by spending Karma for the purpose of Dispelling such spells?

Maybe. There should definitely be a limit for the how far it can go.

Perhaps "Effective Force may only be lowered to real Force." Something like that so somebody with far too much karma can't just nuke a Quickened spell without any effort.

If they're blowing karma on it, I really wouldn't mind. I'd question the sanity in such a fruitless act consider the myriad other things they could blow the Karma on (a magician never has enough), afterall.

I would, however, require them to learn a variation of the metamagic technique. If I were really retarded, I'd even propose calling it Slowening.

That's the thing, Dispelling is a non-initiated ability any magician capable of using Sorcery has. You're essentially looking for a way to burn Karma so as to improve the Dispelling process which, while related to Sorcery, is wholly different from Quickening and Spellcasting.

But it would only work to reduce the effective Force (in terms of Dispelling) of Quickened Spells in the same manner than one can burn Karma to increase the effective Force (in terms of Dispelling) of Quickened Spells.

It's still contrary to the nature of Quickening. I mean, Tattoo Quickening is in the same league and even it requires its own variant metamagic technique. Might as well go the full route with it and allow the technique to be used with any dispelling technique, not just against Quickenings.

So what would such a Dispelling Metamagic look like then, and what shall we call it...

Dithering
When dispelling, a magician can spend a free action to use the Dithering metamagic, which reduces the effective force of a targeted spell by one for each karma he or she burns. The spell's force can be reduced to half the original in this manner.

Actually, if turning it into a metamagic, I'd probably start off with something along these lines. My ability to be graceful with my words have left me tonight, so please excuse my wording here...

Unweaving
Your ability to destroy enchantments has grown by leaps and bounds. When using the dispelling or banishing specializations of Sorcery or Conjuring, lower the effective Force of the target by one-half your Grade. You can further lower the Force by expending one point of Good Karma (or, more likely, by burning Karma Pool) per point of Force. The standard rules for minimum TNs are still in effect, so lowering the effective Force below 2 is generally a waste.

I'd test that out a bit and see how it worked, then adjust accordingly if needbe.