Hello Francesco, community.
The Mozart concert was really great and the orchestra played my piece really well.
That being said, I must second William when he says that common live performances hardly come close to a good mockup. Alas, for some reasons, I didn't install my H4N to make a live recording. But I payed extra attention to what I was hearing.
First and biggest problem is : the orchestra had only two rehearsals before D day. Apparently, more is practically impossible (at least in small Belgium). I heard clearly the piece increase in quality at every iteration, the conducter was really fine, understood everything (ar... nearly) at once and was giving nice directions. Yet, the day before the concert, there were still bugs. So the real spirit potential of the piece (if it has some) never came out until the very concert, and then, I can only dream about how much better it would have been with only two more rehearsals. Funny, at this stage, to hear that when they play the Mozart Requiem, the orchestra immediately sounds like 5 times better. Of course, all the musicians know the music of Mozart, know the Requiem by heart, and know perfectly how to play it, and the difference in SOUND is tremendous.
Here is the right place to say that this is, in my opinion, the limitation of mockup performance. Even if you, daw programmer, do have some knowledge about how exactly a single instrument performance should be, you nevertheless are stuck with what was recorded in the library, no matter how finely precise that work was done. This is clearly a limitation. And one positive side would be to, as a composer, to compose intentionnally with exactly no more than what your libraries can do. This is a nice matter of discussion : libraries considered as a musical instrument per se. But that was not the meaning.
To say it shortly : a good library well programmed can not beat a bunch of dedicated top musicians well prepared and giving their full education in an inspired performance, but it outperforms easily an average third sight reading.
I payed tight attention to the mixing engineer point of view (hearing rather), and I was surprised how, in a large church orchestral live concert, there is WAY more wet reverb than what you would consider decent in a stereo mix. Even I who do like lots of wet (I am often told I put too much reverb on my mixes) I clearly noticed that there was an enormous amount of very long reverb at the concert. Curiously, at the same time, the direct sound from the musicians was WAY more sparkling and precise, and with a kind of intimacy that makes you feel very close to them. And finally, the stereo stage is nearly inexistant : you don't hear clearly first violins 5 meters left of the celli.
That being said, I am convinced that the ideal audiophile listening experience in one's living room is NOT mimicking the live conditions of the concert. Human brain adapts really well and really immediately to a whole range of situations, and the feeling "this is right and nice" depends on many factors, some of them not strictly related to the acoustics.
Sorry for being long, but that was the opportunity. The day of this concert of Mozart Requiem and Mozart aux Enfers (my piece) was the day after the Paris terrorist attacks. The church was full of people full of compassionnal feelings and terrible fears. The harmony was nearly touchable. This you don't get behind a computer monitor.
Anyway, as I said, this kind of experience will not be given many times to me. So I'll say it again, thanks to VSL (and all other producers) for making all this possible.
Stéphane.