The espresso machine I use at work has a pre-infusion step. I've been wondering if the volume of the pre-infusion affects the shot, especially the pressure of the main extraction.
I was actually reading an article on this yesterday. There is no added volume during pre-infusion. All the pre-infusion step does is wet the espresso fully to prevent channeling.
The article I linked actually shows that less extraction happens during pre-infusion. So to answer your question, it does affect the shot, but not the volume.