Although I haven't tried playing an evil conjurer before, I have played quite a few goodie conjurers. From my observations, Conjuration power levels are as follows:

MANA LEVEL
900+ Lesser Archangel/Transplendant Archon
600+ Terrible Angel/Radiant Archon
400+ Radiant Angel/Blazing Archon
Less Winged Angel/Luminous Archon

With that in mind, there are other factors involved. In a previous post from Daevren, he mentioned that when you put extra mana into a conjuration, it either goes all to increasing the level of NPC, OR all goes into increasing duration. Other factors to consider are:

Density of the Veil:
A thick veil drains away the extra mana you put into conjuring, so a 600 mana conjuration might only contribute 500 mana, and result in a radiant angel instead of a terrible angel, while a thin veil contributes extra mana. So an 800 mana conjuration might result in a lesser archangel instead of a terrible angel.

Remaining factors to consider:
Conjurer's level
Conjurer's Charisma
Conjurer's alignment vs. alingment of NPC
Conjurer's ethos vs. ethos of NPC


Based on what I've seen from watching other people play neutral or evil conjurer and conjuring demons and devils, I would say all these factors are equal across the board (only differnce being that the CLASS of demon conjured is not based on mana output, whereas the class of angel conjured IS based on mana output).