That's the question that bugged me for quite a long time. With my spies checking AI planets I finaly got an adequate answer, and updated GalCiv II Wiki - Difficulty Level page.
That would explain why I haven't been seeing a big difference from Obscene->Suicidal like I see with Maso->Obscene. The bump from 200% to 300% bonuses is big, but 300% to 400% isn't so bad.
Another thing I noticed that seems to be a bug: I'm capturing AI planets on Suicidal and getting really big research bonuses. For example, I've got a new planet with two Invention Matrices on it, and "Planet details" doesn't particularly document any bonuses. 312 research.