In the Binding of Isaac, if a stat has all of its bars filled when I look at the pause screen, does an item that raises that stat for a single room cause an effective increase of that stat past the maximum or does it just round to the maximum? In particular, if my damage stat is at maximum, does the Book of Belial cause me to deal more damage per hit?
I would guess that this could be tested by counting the number of shots to kill a particular boss with and without using the item but I am usually too busy not dying.
Best Answer
According to the wiki, the only item that seems to be able to raise your stats above the maximum is The Sad Onion, which doubles the firing rate no matter its current value. I have very little personal experience with that item, so I can't confirm or deny this.
The stat that I find easiest to max out is damage, as I've often managed to get quite a few damage enhancing items in a single run (which would have put me over the maximum looking at the bars in the pause screen). I always seemed to hit the same cap, no matter how many damage enhancers I picked up.
In light of the above, my previous version of this answer indeed states that the item is useless once damage has been maxed. Further research, however, proves that I was mistaken. To test this, I played with Judas, picking up both The Pact and Pentagram, which maxed out my damage. I then engaged the boss Gish. Here is Gish's health-bar before I started measuring damage:
I then shot Gish three times, bringing him down to this:
The difference was 11 pixels. Then, I used the Book of Beliel and shot him three more times:
This time, the difference was 15 pixels.
So as a matter of fact, the Book of Beliel can and will take you above max damage.
Sadly, this experiment doesn't give a single conclusion about the multitude of other ways that one may try to raise his stats past the maximum, but strictly speaking, the answer to your question is yes.