While I'm not necessarily opposed to the idea of being allowed to use your 'complexity' spec as if it was a 'power' spec when engaging in Thaum-as-Evocation, I can't find anything in the mechanics that allows this. Complexity bonuses allow you to increase your Lore when casting Thaumaturgy, which helps not at all when doing fast Thaum thus requiring a high Conviction.
I think that the better solution, however, would be to allow 'power' as new Thaum specialization choice only useable in conjunction with sponsored magic. Alternatively, you could argue that your sponsored magic, in effect, creates a new 'element' (kind of like adding a trapping to a skill), that does everything that Thaumaturgy(Necromancy) does. This would allow you to invest in specialization for your new Evocation element. Or, at least, for the power aspect of it, since you'd probably want to use your Thaumaturgy(Necromancy) control bonus, which would work for both Thaum and Evocation.
In either case, the above ideas would be house rules. To my understanding, there is no 'power' spec that applies to Thaum spells, even cast as evocation.