FWIW, that type of valve is adjusting the volume out, not the pressure, which is fixed. The pressure is the same whether you're feeding a soda straw or a fire hose, but the fire hose can provide much more volume. To make a shower head accelerate the water, there has to be a restriction, and the supply must be more than the restriction will allow to flow through to get the maximum velocity of the water out of the head (Bernoulli principle). Volume and pressure are related, but not the same thing when you're talking fluid flow.
For a little more, on that Delta rough-in valve, you can get a thermostatically controlled valve. Still two handles, but one sets the temperature, and the other the volume. Winter/summer, as the cold water temperature may change, your thermostat setting doesn't have to change...it adjusts the hot/cold mix to keep it the same as the inlet temperatures change either as you start to run out of hot from the tank, or the cold gets colder. On mine, since the air temp is usually a little cooler in the winter, I like to bump the thermostat setting on the valve a little higher, but it's a seasonal thing and personal preference...otherwise, the actual water temperature would stay the same season to season.
