Hi,
I´m using a Mac Pro 8 core @2.26 GHz with 24 GB RAM and I´ve just bought Vienna Ensemble 5 with MIR Pro and from what I experienced so far it runs very well on my machine and I don´t see a reason to get a slave for it. I haven´t had the time to use it with a bigger template because I just don´t have one yet :) I´m in the process of building my first serious template at the moment and I also use Spitfire Percussion + Harp as my main orchestral percussion and Harp and Cinebrass (incl. Pro) as my main brass. I´ve tried using LASS in my template, too, but I decided to go the Vienna way for strings and bought VSL Solo, Chamber and Appassionata Strings incl. Extended recently and I totally love their sound. Woodwinds are VSL, too. My first impression when using other libs that have a room "baked" in like Cinebrass or Spitfire in MIR Pro: It works very well. I have their stereo widths in MIR Pro on the widest possible position to leave as much of their original panning intact and play with the distance to the mic at the moment (the closer to the mic the wider their original stereo field) and I play with the amount of MIR room for those libs - I´m still experimenting a lot but my first impression: it´s an awesome tool to blend different libs together.
In VE 5 Pro I decided to put the MIR room sound on a separate bus as well as the enhancing MIRacle reverb that comes with VE 5 Pro. So I'm able to specifiy MIR room and reverb for each instrument/lib seperately and thus give libs that have already some room baked in a little less MIR room. I tried to use the dry mic setting in Cinebrass and Spitfire, too, but finally decided to leave their complete original room/hall signal intact and just gave them some less MIR room.
I can only recommend getting the VE 5 Pro/MIR Pro demo and try it - it´s no rocket science and I was able to get along with it in about 4-5 hours or so.
Marco