I've probably written about this before, but let me just say... I am soooo glad to have (for the first time ever,) an organ with a "transpose" button on it. I have been using it for just about everything! It is delightful!
Now, surprisingly enough (since this seems so obvious to me,) this is actually a very controversial topic in the organist/musician community. Many people argue that the congregation needs to just learn how to sing properly, then they wouldn't have any problems with the high notes.
Actually, no. First, the congregation needs to learn to have a hymnal in front of them with all four parts (soprano, alto, tenor, bass,) and then they need to learn how to sightread their own voice part; the voice part which is most comfortable to their natural range. Well, let me tell you... since that is that most definitely NOT happening any time soon, I think it's safe to say that not everyone is a soprano, and therefore should not be singing the soprano line as written. "Well perhaps," you might say, "we should just teach them how to sing properly." Ok. Have you ever organized an outside-of-Mass musical event that required any participation from Catholics? Ok, well then, you first! (If you can get more than 5 people to come, I'll be amazed; and if you can get them to actually do anything, well... you'll find me buying a lottery ticket on that day!)
So, the solution seems obvious to me. There is a large group of untrained singers, no chance to "teach" them how support their singing properly and breath correctly (etc, etc...) so that they can really sing high, I would estimate that more than 50% of them are not even true sopranos or tenors...so why pretend that they are?
In my experience, people start to complain around high D. C-C is ok. B-B is even better. So, yeah... I've been transposing everything down 2-3 half steps. It's soooo fun. :-)
(With the occasional "ooops! I forgot to transpose that next song back up and am now groveling on a low Ab for this entire song...!")