One more thing -
the 0 encumberance is huge. An Arch Theurg has a bas encumberance of 8, going up to 10 for temperature reason.
So looking at orders like Quickness, Mind Burn(4x), we have;
Arch Theurg (3 astral):
(20/1+8) + (20/2 + 8) x4 = 100 fatigue, 18 per MB
Soulless Version (3 astral):
(20/1+0) + (20/2 + 0) x4 = 60 fatigue, 10 per MB
4 Astral theurg is even worse - it can cast Mind burn twice and get less fatigue than the living one does for once. And with heat effects the live Versions get even worse - 20 fatigue per MB, soulless stays at 10. Net effect is the Soulless Version can cast almost twice as many low fatigue spells before getting fatigued, which really helps out a lot.
also, a passed out Theurg who gets converted comes back with no fatigue, so if he passes out on turn 5 and then dies on turn 6, he comes back fresh on 7 and ready to start casting spells all over again
And communicants who get overused come back as soulless too - they can now lead undead and are good candidates for unlimited Skull talisman summoning.