Any branch can support more than 500 players without issues, as long as you've got the hardware for it.
Not unless the machine you're using PocketMine doesn't have enough RAM and/or CPU power to handle it. PocketMine doesn't take much to run, but to have more players/plugins requires more power.
I don't wish to start a flame war; but I can say with certainty: No. Even with the fastest CPU available (key is single core performance), fastest SSD, lots of RAM, and top of the line network...you will start to see significant lag ramp up over 40 players. This is somewhat less true if you are doing a mini-games server (tiny world, no saving, etc). I am not trying to fault the PMMP team; there are significant network bugs/inefficiencies in the current MCPE App that significantly impact network performance. The sweet spot with a normal size survival world is currently around 30 players. There's no way you can go above 100 and not be in lag city without massive amount of tweaking (e.g. elimination of features, zero plugins, tiny world, etc).
Well, it's improving the "network". It probably does nothing good for you, it's more about the API side of it. That's why it is called api3/sth. Unless you want to know in what way your plugins will break later, they are of no interest.
api3/network is currently home to a bunch of heavy refactors to make unit tests more convenient and to make protocol data auto-generation easier.