We have now bumped up the default FPS for our L4D servers to 250, to match our other games. We've also enabled the hyperacceleration option on our order form in order to allow customers to order servers at 500 FPS.
Both changes are made possible through the use of a Metamod:Source plugin that clears the fps_max variable. This plugin is installed automatically on new servers; if you already have a L4D server with us, you can have our system install and activate this plugin for you through an option near the bottom of the Easy setup page in your control panel.
Regardless of the FPS used, L4D servers still only run at a tickrate of 30, just as Source servers run at a maximum tickrate of 100 and TF2/DoD:S servers run at 66. The difference between 250 FPS and 500 FPS is fairly minor, so we recommend using a standard 250 FPS server unless you need to truly maximize performance.