Sounds like a dry system with some form of mechanical actuator. I have used several kits from NX Express, NOS, Top Gun, and Cold Fusion; I have never really heard of this pressure injection system associated with a gravity feed system because the only force exerted on the fluid is very small from gravitational forces. It takes some form of pumping system to create the pressure needed to matain a proper mixture at the nozzle.
Send me a link or something to read up on this pressure injection system. I have heard of using water injection on many nitrous and turbo charged applications.
A typical industry standard for a nitrous system is to use a fuel pump designed to operate between 4.5 and 7 psi with a flow rate of 0.1 gallons per horsepower at 6 psi