Hello,
1. Currently streaming isn't available, but it's mainly because I've never needed it personally. Also, I think someone said he was going to write it.
Network features are available in standard Python packages like socket. There are also more complex frameworks like Twisted.
2. Some people have problems building it on Windows. I'll try to upload a Windows binary one of these days, when I have access to Windows.
PySFML is as portable as SFML itself.
3. Emacs and Vim are very popular among Python programmers, but it takes some time to use them efficiently. You don't really need any advanced feature from your editor for Python, so feel free to use whatever editor you're comfortable with, and learn more complex tools later. I've used PyScripter for some time, since it's included with PortablePython.
4. Technically, python programs are .pyc files that are automatically generated when you run e.g. python main.py. Python translates the source to a faster binary language, then executes it.
I expect the speed of SFML and PySFML's classes to be roughly the same, as they are both written in C++ (well, my binding is written in Cython, but it gets translated to C++ before getting compiled and I didn't see any performance problem so far). Your Python code will be slower, and there are also some things that might cause problems for performance-intensive applications. For example, Python objects have to be allocated dynamically, while in C++ you can often create them on the stack.
Users will have to install Python and PySFML, unless you provide them with an installer.