I created an antipope in Scotland to deal with an untimely excommunicate during the midst of a rather large rebellion. After the rebellion was over, I used the antipope to excommunicate the rebels and execute them, which worked out quite nicely.
I did some reading and it seems that my income should have gone up significantly. It didn't go up at all. Even over a ten year span, it never changed unless I got a new tax building or county. I couldn't figure out to determine if Bishops had a higher opinion of my Antipope or the de jure pope. I didn't see anything in the ledger.