A simple, "no, we don't support that" is fine. The fact that you even suggest using "multiple monitors on a single PC" with your little lecture about "modern graphics cards" shows that you haven't even the slightest idea what you're talking about. I currently run five 4K displays + two panel displays across 4PCs. Running Prepar3D v3.4, every single one of them is CPU-bound running the latest and greatest i7 CPU -- and overclocked to boot.
I'm sorry but that's not that case. Or, more precisely, in your situation, you are BOTH CPU-bound AND GPU-bound.
This true for everybody running P3D if you're running at any significant level of detail and especially if you're running 4K displays. FSX and (to only a slightly-lesser degree) P3D depend upon single-core performance; the main rendering thread it both can only us a single core. This is a well-known fact and literally thousands of explanatory posts in a dozen sim forums have been written about the way the two simulators work.
It's not so simple. If you runs with ALL settings maxed out, there are SOME settings that are affecting the CPU, and some settings that are affecting the GPU. If you run with everything maxed out, with five 4K monitors, there's no network that can save you from the extreme impact on the GPUs.
Could I downgrade to lower resolution monitors, run fewer displays, and get decent frame rates on a single-PC simulator? Yeah, I could.
Well, you probably should.
But doing that just to get GSX eye-candy isn't worth it. Looks like I wasted my money on the single license I purchased.
You are making it sound as if it's just GSX that is preventing you to use your setup as you would like to. As if each and every 3rd party software out there was able to syncronize itself over a network. I was discussing network traffic, if GSX had to sync its status of all its running animations, now imagine if every 3rd party software running the sim would need to do that, and it will be very likely to have stuttering, which will be even more noticeable on big screens.
With a single PC, you don't just get 100% compatibility with EVERY software, instead of having to ask to each single vendor to modify their software to support your setup, and that's not going to happen. And, you can upgrade more often, up to a point that, newer GPUs will eventually be able to drive your five 4K displays (maybe not now, but in a couple of year's time) easily, but with 5 machines to update, it would become a very costly proposition.