Dominik,

Even what you are saying is right from the electronic point of view, this is not (always) the case in real life.
EX1 does a good job with dark. But on most of camcorders the sensor is already amplified to the noise limit. Adding gain will then cross the acceptable limit. Then, you will end with a lot of noise and even more if your camera is badly color aligned (back balanced).
I'm pretty sure that you won't use a +9 or +12 gain even with what you are saying here. Else, at some point, you won't be able to have a good picture, whatever you do in post.

Going to a 10 bits codec will not remove the grain, noise or compression artifacts from the source. It will just enable you to have a lot more room to tweak the picture.

Once again, it all depends on what you want to get in the picture and decide which part to advantage. Use gain to get detail back in dark area, don't to sacrifice dark area but don't clip the whites...

Finaly, in some situation, you will not be able to get any good picture whatever gain you set. The key, then, is to control your environement... maybe buy a torch, or have you friends light some candles :)

Why? Because in real life the ex1 isn't an electronic device?
The question is: Why should I end up with less noise, if using gain in post?
Of course I do! Take a look at this clip:
http://www.youtube.com/watch?v=o9ChXxVV-ww&fmt=18
All shot with +18db, shutter-off and f1.9.
Of course there's a lot of noise with high gain, but there's nothing I can do against it, if I don't want to underexpose.
FCPs internal processing doesn't care about the bit-depth of my footage. It will always convert it to its internal bit-depth before processing, so there's no use of wasting my time and harddisk-space with converting to ProRes before editing.
That's exactly what I was trying to say. If you need the whites don't let them clip, but if you don't or there aren't any don't hesitate to use as much gain as necessary to get a well exposed picture. Use gain as your exposure-tool if the others (aperture, shutter-speed,...) don't suffice.
That of course is true. But you can't always do that.

Because each device react a different way from theory. The EX1 is close to it, but....

I can see 2 things here :
- youtube encoding is so lame that you even can't see noise from a 18db gain :)
- on this kind of shoot you can easily get rid of the noise in post, as the important things are the bright things. Crushing the black (increasing contrast) help as most of the shadows are just... shadows

Maybe you could provide some footage from the camera without encoding so I can really see what the noise is at +18db ?

Once again, it all depends on what you want/need.

To get back to the first question, I'm not sure a high gain settings will give good footages in a church, or at least, a church like I know of in France, with candles, some lights, "smoke" or fog...
When I'll have time to get back to a church i'll try every gain settings and see, but i'm pretty sure I won't like the look of a high gain in a footage with a little girl praying, like the one I made last time.