I heard this question a few times following the Easter services. During such a celebratory service, singing some of the most rousing and beautiful hymns of praise with a full church, I did let the organ pipes roar beyond what I would do on a typical Sunday, of course. In the last verse of the opening hymn ("Jesus Christ is Risen Today"), you heard the organ at approximately 90% of its theoretical full volume.
Why only 90%? Well, first I'll ask you: wasn't it loud enough already? Second, not all pipes on an organ can be played at the same time and sound good together. The salicional and vox humana don't get along well with the flutes and diapasons (to put it in geeky organ language).
Also, our unique mix of pipe organ and electronic keyboard means that I can add in as much volume as the lectern-side speakers will pump out. On Easter, I was using the electronic pipes mostly for the 16 foot and 32 foot pipe sounds - in other words, that low rumble you might have felt in your chest during some of the hymns.
Given the contraints of what sounds good, there's really no need to go beyond 90% capacity of the instrument, and we probably won't hit that mark again until next Easter or at least until Reformation Sunday or Thanksgiving!