Yeah, I've done it. Also, that line about "since the Intel 6X/7X chipsets" is way, way off the mark. You need to have working IOMMU/ACS for VFIO to be usable, and even then it's not likely to be very stable. Pretty much the only boards that really got ACS/IOMMU working 100% were the X79/X99 boards. Lots of others (My Z97 included) say it's working, and will work for a quick setup and a GPU-Z screenshot, but will crap the bed and take the host down five to ten minutes later.
Much like wheelies on a motorcycle, GPU passthrough is exciting and impressive, but also like wheelies, it's hard to maintain and requires a great deal of maintenance or you'll hurt yourself.
When you add in multiple GPUs and multiple USB cards to handle guest keyboards and mice, the overhead goes up and the chance of error failure does as well.
If you'd like to know any specifics, shoot them my way. I've had Windows, MacOS, and Linux guests running on my Linux host. I'm not currently doing any passthrough, for the above reasons, but I can easily reconstitute my setup if you need to see something or copy parts of a config.
I'd recommend subscribing to the vfio-users mailing list if you're serious. A month or two grazing that list and the problem reports will give you a really good idea of what you're in for.https://www.redhat.com/mailman/listinfo/vfio-users