Should I replace my VSTi out channel sends to 7.1.4 alg reverb with MIR or is it better to add one on the instrument buss?
MIR isn't really meant to be used as "overall reverb" (although it can serve this task pretty well, too). The basic idea is to have one "MIR Icon" on stage for each individual, dry instrument, or at least for uniform ensembles like "all 1st violins" or "all French horns". In your case with hundreds of tracks I would suggest to create groups for those sources, adding MIR instances there: MIR is a convolution based engine which tends to be CPU-hungry, especially in a 3D-audio environment ....
Enjoy MIR 3D! 馃槉
/Dietz - Vienna Symphonic Library