Interesting. His contributions seemed about equal with Paul's up until Lick It Up, and then again around the time of Revenge. Especially Carnival of Souls. I don't disagree with the self serving bit, but I just figured he was more involved in Kiss during those times. So that's a surprise.
He was still mostly present at that point, but from Destroyer onwards they started functioning less as a unified band, with less collaboration on each other's songs, and more outside musicians, especially for Peter and Ace.
And even though Gene was always contributing songs, they were never as polished as Paul's, so there were less of them on the albums. Just check out the Love Gun demo, and aside from the guitar solo, it's identical to the studio version. Paul even played bass on that one.