You can find them on this site or you can visit gnomelook.org.

First of all in Linux there are different types of themes:
1. Pixmap themes, based on the pixbuf engine.
2. Themes based on various engines, the most popular are clearlooks, ubuntulooks, smooth, glider, industrial, and of course Murrine.
Murrine is an engine, written in C language, using cairo vectorial drawing library to draw widgets.
Then you need to install themes in order to see what murrine can do! :)

Since the engine provides only a library, then you need themes (that are simple text files) which use Murrine engine as their dependency.

Yes, much faster...

Yes and no :) ... Using a squared theme the engine seems to be the fastest cairo engine ever.
However if you are using a round theme the engine isn't fast as before but the speed slowdown isn't so big. Few benchmarks notices a speed between clearlooks and ubuntulooks using round themes.

Yes... it is the Murrine special feature... check out the Configurator project page.