I'd rather not pay good money to test it all by myself and despair. As we speak I am finishing mixing a huge piece (53-stave (not track) score with 2-4 voices on each - excepting a few percussion staves) on an iMac 2.16 Core2Duo 3 Gb RAM with all the orchestral and choral sounds coming from inside the iMac itself(!) in Logic 8.0.2 using Space Designer. So I am pretty surprised to hear that on modern and huge computers - even with MIR - one cannot mix tutti...
I was planning on getting the latest iMac i7 soon, throw 16 Gb RAM in it and the biggest optional SSD for some of the libraries, and I thought I would be comfortably mixing complex musical material, for the aforementioned piece is not Symphobia held pads or sampled runs etc. It involves anything from 5-11 different voice-groups/lines of polyphonic material blown orchestrally, with further subdivisions/transpositions etc. at a moderately fast tempo, i.e. lots of notes, with moderate automation lines for each of the tracks. My future plans would be to supplement this i7 iMac with a moderate PC for MIR and the libraries and do the sequencing and mixing on the Apple; does this sound unrealistic?