MirPro is an amazing product. There is quite literally nothing else on the market that does exactly what it does and with the level of quality. The technology it uses with ambisonic encoding of stage positioning on numerous real world locations, is matched by nothing else.
Spitfire stuff has its own stage positioning and if you're happy with the ambience built into Spitfire, then you don't need Mir. MirPro is more about having total flexibility and adjust the room as you want it. You actually need dry, mono samples fed into Mir for it to do what it needs to do. If spitfire does not include close dry samples without panning, then you are already going to be a little compromised, though you can certainly find a way to dial back the reverb of Spitfire and you can also mono-ize it using other plugins, before sending to MirPro... But MirPro ideally works best with dry, closely mic'd instruments, without panning info.
I personally love MIrPro and I bought all the room packs. So I have a few dozen rooms to choose from, then after choosing the room there are a lot of parameters I can use to effect the way the room sounds. You have one main mic array, which you can move around to any location in the venue, and you can choose different kinds of mic arrays and configurations, with different mics and mic response too.. And there is a secondary main mic array that you use also to place further back in the venue and capture the lovely reverb of the space. then you can position the players around on the stage wherever you want, and it basically all goes through the ambisonic engine to create the panning, the distance, the width, etc of each player/section, as well as blend in the reverb of the room through the main and/or secondary mics. You have almost as much freedom and control as a real engineer would have if they were recording the musicians at the venue for real.
I bought MirPro because I also bought VSL libraries, which were all dry and needed something like this, but I have been so impressed by MirPro in general, it might be the best product VSL makes actually IMHO. Its truly amazing. But its not inexpensive and its not going to sound gorgeous out of the box with the click of two buttons. You will have to spend some time learning how to set it up and get the sound you want. Its not that bad once you get the hang of it, but numerous people have gotten it and then felt unable to get the sound they wanted. That is in contrast to something like Spitfire, where you just turn it on and it sounds amazing. But you have very little flexibility to change the room sound of Spitfire, so there is that.
I started out with just MirPro, then ended up buying the Synchron room pack because everyone said it was their favorite, but in the long run I just ended up getting them all and truly there are gems in every pack. There are some churches in there with gorgeous blooming reverb tails to play with and useful spaces, and also some small recording studio style rooms which you could use for any purpose at all including rock mixes or anything else.
MirPro is a killer product, but its not for everyone. In order to make use of it, you'll need to commit to the process a bit see where it gets you.