If the plugins you're using support what you're saying this could be nearly lag free indeed.
In my experience, however, I found that splitting up the gamemodes in different servers is not only easier, but also eliminates any lag being transferred from one gamemode to another.
The thing with factions is that when 5 players run in a different direction, a lot of new chunks are generated. No matter how strong the server, if 20 people run in different directions, there will be server wide freezes of about half a second. This can really ruin pvp battles.
PaperSpigot has some world multithreading, I think. Not sure. But if so, it would be better than Spigot assuming all plugins can work on it. I found one of my own plugins, NubesHG, not to be compatible because of one little feature. When someone told me it was incompatible, I changed it, and now it's compatible. So if any plugin doesn't work on Paper, you can always ask the developer.
To summarize, ONE server could work well. But it really depends on your standards. I myself have a zero tolerance policy for any tps less than 19.
