I think some people are confused at what MIR Pro and Mirx are...
MIR Pro is the completely flexible and fully customizable convolution reverb engine, which allows placement of instruments in a drag and drop style UI that displays the venue in a graphical type representation of said venue. Six rommpacks are available seperately from the engine (the engine itself does nothing without one of the six roompacks). MIR Pro 24 is a less expensive version of the engine that is limited to 24 channels, but otherwise, is the same as MIR Pro feature wise. MIR Pro and MIR Pro 24 are available inside Ensemble or as a VST3 plugin.
MIRx is a significantly less expensive option without the fiddly-bits that MIR Pro offers. Unlike MIR Pro, only 5 venues are available (as opposed to 6 for MIR Pro) and it has a very limited feature set, namely, MIRx ONLY works with Vienna sounds and only offers a few preset locations for an instrument. MIRx is only accessible inside Vienna Instruments when viewing the window in Advanced mode under the reverb tab. Some might argue (including me) that the major advantage of MIRx over MIR Pro is its ease of use (enable it and it just works) and of course its price vs MIR Pro.
MIRx is around $100 per venue (with a total of 5 venues) and MIR Pro venues are between $100-$200ish a piece, and the engine is about $300 for the 24 channel and $650 for the unlimited channel version.
Hopefully this helps, as I was reading through the thread and people are using MIR and MIRx interchangeably, and they are sort of related, but two completely different products.
My hope is that the MIRx venues I have purchased do not have to be purchased again for use with Synchron, that would make me sad...