If AV requires a dedicated PC, then it's out of the question for me. I would rather spend my pennies on a Bricasti! However, I don't care for real-time rendering: I always work without reverb during mock-ups, and add it at the end - easier to hear the articulations this way. Imagine you find a good setup with MIR, you know it sounds good because you tested it on exports, then you wouldn't need a dedicated PC.
I agree with Fritz: paying more for MIR and use it for any instruments! Using MIR with VSL and then trying to find a corresponding patch with Altiverb for other non-VSL VST's seems awkward! Specially if the heart of MIR comes from AudioEase technology.
(I wonder how much it will costs, with all these years in development, the relative small number of end users with only VSL VI's... but like always, VSL will come with an elegant answer [H])