In full disclosure, I work for GEIGERRIG. Thanks for your interest. I know that it can sometimes be unpalatable for companies to post on blog sites. I hope that I am not overstepping my bounds. I just thought it was cool to see the Sawyer - GEIGERRIG configuration. That said, I would be happy to answer your question about the pressure. Again, thanks for the interest.
[There is a good video on Youtube: "Geigerrig Hydration Pack: Constant Pressure". It demonstrates much of what is discussed below.]
GEIGERRIG has a patent pend. pressurization system that is very simple and provides a very reliable, consistent and powerful spray.
Instead of 2 outer walls of polyurethane, the GEIGERRIG has one additional sheet of polyurethane welded right down the middle. This creates two separate compartments, separated by a flexible wall. One compartment is filled with water while the second compartment is pressurized with air using a small pressurization bulb similar to the one that is used with a blood pressure cuff.
By separating the air chamber from the water chamber, you get a uniform application of pressure over a very large surface area of the water, thus stabilizing the water mass even as the water is depleted from the pack. Also, because the pressurizing air is inserted into a second chamber, this prevents the pressurizing air from ever engaging the drink tube and subsequently depressurizing the system. When air is inserted directly into the water chamber, it creates a scenario similar to what you would experience if you filled a balloon half with water and half with air - The water mass slams around inside the balloon. Also, it is nice to keep the pressurizing gas or air separate from the water - its cleaner. Kind of disgusting when people blow air back into their hydration pack and then offer you a drink.
Bottom line, you get a very consistent, reliable, clean, high pressure spray this way.
Accordingly it facilitates in-line filtration, sharing water without a requirement to suck on nasty bite valves, and spraying water for field hygiene and cleaning off gear.
It is a very clean, reliable system. The extra sheet of polyurethane and the pressurizing bulb are very light weight and generate very little in the way of additional cost.
Additionally, the GEIGERRIG pressurized system is configured so that it can be turned inside out for cleaning in the top shelf of the dish washer or for field rinsing and hanging to dry in a tree.
Lots of info at www.GEIGERRIG.com.