Illusion is a good set, and Dark Affinity is also a very good set. But I think that you're overly interested in cute numbers, here... the main defensive power of Dark Affinity is Darkest Night. You do not need to go to such defensive extremes on pure +Def when you have such strong -ToHit debuffs (and more from Dark Servant). Without Power Boost, this build will still put out great Defense and -ToHit, making practically every enemy start at their to-hit floor. So then hitting Power Boost is not going to add anything to the defense, except for protection against additional spawns.
I think that you're better off when taking such a strong defensive build to focus on any actual weaknesses, and for that reason I'd go with the Psy Mastery epic in order to get Indomitable Will. Alternately, go for more offense via Soul Mastery. But Power Boost on a build that already floors enemy to-hit chances without using it, seems to me to be unnecessary.