This is why I wish we had a better-referenced Wiki... so from the name, I'd guess logicillion applies logic to the Suvian-ness to make it go away. That suggests that giving it to an unreformed Suvian would essentially have a milder version of Reality Room effects.

-Unnatural abilities would be weakened.
-Characters who liked them for no reason would go off them.
-Impossible colours and the like would be muted.
-Impossible biology would start to be rejected.

&c &c. The best place to use it would be when something has happened in the story that would directly debilitate them - for instance if the Suvian has had a transplanted arm grafted onto them, logicillin would likely cause immediate rejection.

Alternately, maybe it acts as a sort of internalised Reality Room - it helps ex-Suvian agents by making them see/feel the illogic of what they were before. By that measure, you could use logicillin poisoning to shatter a Suvian's worldview. Imagine them falling to their knees in front of their designated Lust Object, yelling, "Why was I even after you? You're not interested in meeeeee--!"

And then they get shot in the head. Because PPC.

hS

I am... not sure. But here's my take on it:

Yes, Logicillin might make a Sue easier to defeat. It should definitely have funny side effects. However, within a Sue-warped canon, a sufficiently powerful Sue may be able to shake off the effects. It's hard to say, and I'd take it on a case-by-case basis on rule of funny (or rule of drama, if you're Iximaz. :-P)