Just posting this follow-up because I thought it might help others...
There are stage plots of the Synchron recording session published on the website, which show where the sections and microphones are placed. Knowing that the Synchron stage is 30m wide and 17-19m deep, it is pretty easy to derive the original distances and spacing between the microphones and the sections.
It looks to me that the 21ms delay in the string presets (as an example) are right on, as Bernd has already suggested, because the distance between the MAIN L/R and the string sections are between 5-7 meters (so a good delay variable to use as a starting point would be 15-21 milliseconds).
Additionally, I did some tests in my DAW, and confirmed everything that was suggested in this thread. The microphones are clearly time aligned, meaning that when you disable the delay, or use 0 as a variable, the transient attack of the samples are all aligned to the same starting point. The one minor exception to this is that the relation between the MAIN L/R and the CENTER of the Decca tree were time aligned as a group, for the purposes of maintaining the spatial relationship of L/C/R.
QUOTE: "And I think it is a very wrong decision by VSL to avoid delay while playing on the keyboard."
I was of a similar mind until I understood what was going on. Now I have to respectfully disagree with this. Time alignment is the right approach because I want minimal latency as a starting point, and it is easy enough to add the delays back in to achieve whatever room layout you'd prefer, including a preset that approximates the original recording layout/timing. I wouldn't want to be in the opposite situation where I was trying to compensate for baked in delays and I was trying to remove them.
And lastly, a note on phasing, because this is something that I've struggled with in the past. To avoid phasing, you have to play with the channel delays and the phase button in the mixer just like you would in a real recording session. These are real audio signals of real microphones and phasing is an avoidable physics problem. This is an absolute requirement unless you are only using a single microphone. Additionally, you have to be careful of how you are routing signals in your DAW. For example, if you are routing a dry signal then using a send for a reverb with a less than 100% wet mix, this can cause phase issues. Make sure to use 100% wet reverbs on FX sends and be careful of any FX that might be splitting and recombining signals. Finally, make sure your MIDI setup isn't double triggering notes. I use a ROLI Seaboard and it double triggers MIDI if it isn't configured for the VSL Synchron player correctly. These things can also cause phase issues!
Regards,
Jason Todd Shannon