SunRa re-interfaces a big subset of PHP's functionality, and additionally provides a different syntax and emulates a few things not in standard PHP (most notably type checking). Some developers like the pythonesque syntax, some do not. Some like the type checking, some do not. It's up to you to choose the right tool. For a quick script PHP might be more convenient, for a large application SunRa's compile-time checks and clean API will probably add to the quality and maintainability of the code.
Check out the features page for an overview of SunRa's functionality.
There isn't any! The generated PHP code is optimized and inlined. In fact, you should notice a performance boost compared to an equivalent hand-written PHP script. (Although in most cases you will not be able to tell the difference without benchmarking.)
The 0.x versions of SunRa are written in python 2.5 on GNU/Linux. They will probably run under python 2.x, and possibly on other operating systems without adjustments.