Stay with Apache and remove not required modules. Specially since - like on Litespeed's example configuration, using Apache's prefork MPM and then comparing this non-threaded against threaded webservers is well - not really compareable.
Specially with FastCGI worker is the right MPM to use.
And Xcache and other PHP Optimizer's are maybe a bit more placebo-ware than a real help in the most cases.
Just saying - by changing some software there won't be maybe much speedup. Do benchmarks yourself and see what fits your needs best, not just Webserver related, also PHP, MySQL and whatever else is running + the OS of course