Ok, first of all, here is a compressor map for the GT2860RS Turbo:
The Y-axis is Pressure Ratio and the X-Axis is Air Flow. The islands are the efficiency. As you can see, the efficiency is not simply a function of pressure ratio.
Pressure Ratio is how much the turbo is compressing the air relative to the inlet air pressure. So, at sea level where the ambient air pressure is roughly 14.7psi, a 1:1 pressure ratio will yield atmospheric pressure (0psi of boost) and a 2:1 pressure ratio is 14.7 psi of boost. To calculate boost pressure, multiply atmospheric pressure by the Pressure Ratio then subtract atmospheric pressure.
Remember that boost is simply a measure of restriction. The X-axis on the compressor map is the one that matters when looking at horsepower. Lbs/min of air directly correlates to lbs/min of fuel you can add. Fuel plus air = power.
Now, back to the efficiency question. Since the islands represent efficiency, this turbo is most efficient in the middle island. That said, if you think about setting a boost pressure (horizontal lines) and rev the engine through the range of the engine, the efficiency is going to go from low, to high and back down again at the same boost pressure. Now, if you want to hit the peak efficiency through the rev range of the engine, you'll need to run between 7.5psi of boost up to 19psi of boost. The sweet spot for this turbo is right around 15psi of boost. That will give you peak flow for the turbo while keeping it efficient.
In the case of a Twin Turbo setup, the flow rates are added together and boost pressure for both turbos are the same. So, to get the most out of a twin setup and run it as efficiently as possible, you can flow up to 70lbs/min of air at 15psi of boost. The rough rule of thumb is that for every lb/min of air you flow, you make 10hp. So, that's roughly 700hp potential for a pair of GT2860RS Turbos.
Pressure ratio is not additive but air flow is.