I dig it :)
How about something like a loss of effective spellcaster level equal to number of corruption points? Certainly this would apply to turn attempts, but dunno if it would apply to the number of spells you could cast or the power of the spells, once cast.
I was thinking of this and... I came to the conclusion that the Evil Sorcerer sword & sorcery archetype, in ACKS terms, is a Chaotic Cleric.
The archetypal Evil Sorcerer wants power, he wants a lot of it, and he wants it NOW. He has no patience for study or for solemn prayer. He will pay any price he needs to pay to get power.
Mages don't have easy access to power. They must dedicate their lives to hard, patient study. This is reflected by their steep XP requirements and neglect of any other character aspect other than Arcane Magic. This is not what a Sorcerer wants.
Chaotic Clerics, on the other hand, advance quickly. They can fight almost as well as Fighters do. They get their spells automatically without having to look for a spellbook or performing research. They get all the spells and do not have to learn new ones. No restrictive repertoire to deal with. They even get necromancy better than Mages - they have Animate Dead as the reverse of Smite Undead as a level 4 spell while Mages only get it as a level 5 spell, and they also get to control (charm) undead, which no Mage can do.
So Sorcerers are Chaotic Clerics.
The Core Rules as-written already mention Clerics having to follow the strictures of their faith. This is easy to shape into what I want - Lawful clerics get penalized for doing evil things with their spells, while Chaotic clerics can be compelled or encouraged by their dread gods to do vile things. Shades of Magic merely formalizes this.
I've always thought Clerics were just Warlocks with a better PR department.
I feel this is basically what the Heroic Companion ceremonialists do.