@wibragg:
Don't press buttons if you don't have any idea of what they are going to do!

Now, isn't that true for anything that has to do with planes?
But seriously, since, according to Albert Einstein only two infinite thigs exist, the universe and human stupidity and he was never sure about the universe, ... well, there is a logical conclusion why not to do things like adding servers to a client package.
I looked a little bit into HLA, but not enough to understand all the consequences yet. I started some of my own experiments, not so much with the goal to build an own MP structure, more with trackers, crew-intercom and maybe some tactical data exchange in mind via http. But due to other work in real life, I never made it beyond a basic prove of concept. So, some work there which is independent from the general MP structure/HLA.
The problem with HLA as I understand it, is, it is exactly that, a High Level Architecture. It basically defines objects (for example a plane or a runway) and relations and interactions between them (as in plane land on runway). That is of course very simplified, but it shows already the basic problem. It is high-level, means, machine independent, just another level on top of the already existing ones. Which means more coding and decoding information at runtime. I doubt, an already slow and overburdened system will go faster by putting MORE work on it.
So, HLA will surely bring some advantages, but as I see it, ONLY if the underlaying levels are cleaned out and streamlined first.
J.