Perhaps Kellhus' interest in the Psukhe liesi n breaking it apart and analyzing it more so than directly wielding it, knowing the inherent Dunyain limitations with it. It's entirely possible that Fane stumbled upon a sorcery approved of by the gods through feeling, despite his intellectual analysis of the World's metaphysics failing to hit the mark.
Given Bakker's comment that the World is conscious and the Outside unconscious, and comments elsewhere that the Gods despise premeditation (seemingly favoring going with emotion rather than rationality in any pursuit), it wouldn't surprise me that the Psukhe, based on feeling, harbors some truth of the Outside independent of Fane's rational, thought-out doctrine concerning it. Kellhus could well have been exploring a way to subvert this, to hide thought as emotion or some such, and needed to pick apart the Psukhe to see precisely when in its process it is either hidden from or approved by the divine.
Could it also be more timey-wimey bullshit? Does the Water, or emotion itself, somehow lie outside of eternity as perceived by the Gods? (The latter I consider less likely, as the Gods are far from blind to motivations and emotional states. "Judge us according to our temptations," and all that.)
All of this would require Kellhus to give a damn about damnation, which remains to be seen.