Well, not exactly what you asked, but I have just done a recording using all of Dimension Strings as seperate players, plus I added in Appassionata and String Orchestra as well, 3 players of Dimension Brass were there along with a large orchestra. I had BFD3 for drums, Kontakt was in there plus Trillion for bass (all on the MIR stage).
Now, I don't run Cubase on this same computer, that is networked, but I always run Sibelius on the same computer, so that was active on the same computer.
I always use one instance of VE Pro, and had a very cluttered looking stage in MIR ! I'm sure the trumpet section would have got hit with the viola players bows !
I don't know if different MIR rooms require more processing power than others, but I was using the Gateshead Sage Hall One, (stereo out).
The system is:- i7 processor (960), 8 core, 3.2GHz, 24GB RAM (Triple Channel), Windows 7, WD Cavier Black hard drives (one for VSL, one for everything else in addition to a boot drive). VE Pro/MIR settings are at default settings, VEP Pro 2 buffers, MIR 512.
Cubase's delay compensation can sometimes get a bit over zealous, as the only "latency" issues I get, is that Cubase can record the MIDI playing early, so it looks like I play before the beat - but I'm sure I don't, but it keeps everything very real.
I do have a theory why my system works so well, without being over spec'ed. Because I play everything in real time, everything is naturally slighty off perfect time, for people that step input or quantise too heavily, they probably have times when everything is playing on a precise beat and that could well put too much demand on the system where maybe 20 notes are having to play at exactly a point in time, with my "real" playing, all of my notes could well be upto 50mS apart, and so giving the system time to deal with it.
My final recording is done be bouncing down each track, so that the final mix is done with all audio tracks in Cubase, but during the recording process - everything is playing at once.