It's a conspiracy to drive miners away from proprietary platforms. I also get a percentage from FSF's donations in BTC for doing this
Now seriously I would not mind someone porting that to Windows as I have no plans to do that soon (have other more important stuff to do - it is not about bitcoin mining only, but also password cracking and adding new algos is a priority over porting to windows. I also happen to have my own life, a full time job and a kid to take care of). However, porting this to Windows is not gonna be easy as the code itself is messy and depends on linux-specific stuff a lot. It was never intended to be ported to win.
Also, no, the performance advantage would be no more than 3% as compared to other miners. With the latest version, I had to focus on correctness and feature support over performance so the hashing speed kinda decreased using the "fast" command-line options. So no way you will be getting 5% more than phatk for example, except on 69xx hardware probably.
Also, it would not run nicely on multi-gpu systems on Windows and the reason for that is that the windows opencl runtime does not honor GPU_USE_SYNC_OBJECTS. Until AMD fixes that on Windows, supporting multi-gpu configurations from inside a single-process application (even multithreaded) is useless as kernel invocations are to some extent serialized and CPU usage is high due to spinlock usage in the ocl runtime.