The gain from plumbing in the Pro 800 is that the line pressure will fill the boiler and the pump will never operate; when using the reservoir, the pump is used only to fill the boiler. Pre-infusion is always a function of boiler pressure, regardless of how the water is delivered to the boiler.
The length of pre-infusion time also affects the volume of water that will be pulled in a shot. I've gotten as much as 45 grams out by pre-infusing 5 seconds or more.
That being said, a finer grind or larger dose or both will reduce and/or eliminate coffee dripping from the PF during pre-infusion. I've been playing with those variables and tasting the shots that are way out of the normal timing scope for pump machines, and it's really rather amazing. I have yet to get a sink shot, even with shots taking over a minute to pull.
Hope that helps...