I understand the desire to see a test in 5.1 .. but as I say I don't have any way to monitor and know pretty much nothing about how one sets that up. Surround is just one of those things that no one has ever satisfactorily described how to use properly, and how it all helps in the world of stereo headphones (which is where most of our music ultimately will end up!). I assume yes, it uses much more CPU power .. but I can imagine my 6-core mac pro being able to handle a good sized orchestra in MIR.
Mind you, because of my desire to have a nice swift workflow and not so much arsing around inside of VE Pro, I prefer to run MIR as a plugin within Pro-Tools, and utilize MIR's capabilites in a straight forward audio mode. This is largely due to the constraints of my studio .. only having the one license means MIR is bound to only one computer. When (and I do say when!) I have acquired a much bigger, and way more expensive studio setup .. and a hollywood film that'll pay for it all :P .. then you can be sure I'll be working in Surround .. with about 6 high end Mac Pro's doing all the work!
As I've previously stated I use my Mac Pro for not only music, but video editing, and 3D rendering as I've just gotten into 3D art and animation. The demands of editing 4K footage (or higher), or for rendering an animated scene in hi-def are considerable. If you've ever tried to render an animated scene, even short, you'll know that even a 5 second scene can take a whole day to render .. even on a good computer. The question most people are asking is .. 'Will a Mac Pro speed up my workflow and allow me to do more?' .. to which the answer is definitely yes.
Is it worth the money? .. hard to say since the more advanced computer users will no doubt prefer to build their own hot-rod system and it's quite possible you can make a similar spec system for less than the cost of the Mac. Mind you, Apple REALLY threw in every high end thing they could. The data throughput in the Mac Pro system has very few, if any, bottlenecks and gets things done pretty damned fast. The only real bottleneck you'll come across is when you attach storage to the unit .. I'd suggest investing an a Thunderbolt 2 SSD drive since that will offer the greatest amount of workflow satisfaction. But even with a decent USB 3 drive things work nicely.
At the time I'm writing this, it's become apparent to me that the Mac Pro must run everything .. the idea of using slave devices is nice .. but not very practical in my studio. For the reason that a lot of my projects utilized different setups and so every time I want to open up an old session it requires me to transfer key licenses, and to reconfigure the setup in Logic to keep up with it all .. which takes ages. I'd rather have it all accessed through one machine. And frankly the Mac Pro handles any and all of the music sessions I have any of my previous computers through.
So what of the iMac slaves?? Well .. they make excellent monitors for my Mac :D (2 x 27 inch LED screens aint' no bad thing). And my i7 iMac is also now my main 3D rendering machine .. since it's going to sit there and do bugger all for most of the time.
And of course there's the main reason to buy a Mac Pro .. cos it looks f***ing awesome :P. Why buy a Honda Accord .. when you can buy an Aston Martin :D