FastCGI can only increase performance significantly if you’re using it along-side a caching mechanism like APC or eAccelerator. While FastCGI in itself makes the site a bit faster, you’ll run into many issues down the road with it if your site grows substantially. Tweaking your php.ini to maximize performance also goes a long way (fastcgi or not) to improving your site’s responsiveness.
You can see the speed of my site (link in my sig) below using a custom PHP5 compile with FastCGI and APC (though I normally use eAccelerator), as an example.
Chips N Cheese - Custom PHP installs and the like!