On the topic of the switch to server-client for even singleplayer:
This is entirely a good thing. Right now it sucks, because they previously had to maintain separate codebases for the client and server. Now that they're all the one piece, work can proceed apace on optimization of the server and communication, without having to stop work on the client and other fun stuff. For modders, it means the same - one codebase for your singleplayer and multiplayer mods. It also means that any mod made properly will work in both single and multiplayer.
There'll be a transition period where everything sucks, and then it'll clean right up.
Also, I have no performance issues whatsoever. Chunk loading is fine. Flew around a bit in creative mode on a new world and never once went off the loaded section.