Short answer: probably not. Especially since you state that you "have a lot of enabled modules and some of them were customized."
Longer answer: Pressflow's changes are relatively small, and hardly break the APIs. However, there are some incompatibilities, most in the area of database-access and caching. Especially modules that knowingly or unknowingly don't play by Drupals coding guidelines, will probably break. My suggestion: just try, if a module breaks: fix it (and file a patch).
But the real question is: are you going to benefit from Pressflow? It is not simply "better". It allows database-replication, such as load-balancing or master-slaves. Do you intend to use that?
It introduces better support for caching proxies. Are you planning to run a squid or some other caching proxy?
It has some small changes in, for example, the area of caching, that may (but may not) help you; depending on your current usage.
My suggestion: first see how to improve performance without Pressflow. Then, once you come across an area where Drupal is of little help, but which is "fixed" in Pressflow, consider changing.