Old-school muscle car guys often talk about flow in terms of cubic feet per minute (CFM). This is volumetric flow rate assuming that the air is at standard temperature and pressure (STP). That's fine when talking about naturally aspirated carbureted engines sucking atmospheric air, but inside the manifold and after the turbo, temperatures and pressures are drastically different. More accurately, we should talk in terms of mass flow rates like pounds per minute or grams per second, which is how compressor maps and mass airflow meters think.
Mass airflow is far more useful because, regardless of temperature and pressure (volume is constant inside the engine), we know exactly how much air is inside. Once air passes the MAF sensor, the number of oxygen, nitrogen and carbon molecules don't change until fuel is added. It's the amount of air inside the engine that makes the power-not the pressure it's shoved in at. Incidentally, when we talk about air/fuel ratio, it's a mass ratio. This is why ECUs are calibrated in terms of mass flow. Knowing exactly how many pounds of air are flowing into an engine allows the ECU to calculate how much fuel to inject: 14.7 pounds of air for one pound of fuel for stoichiometric combustion. If it were a volumetric ratio, 14.7 cubic feet of air for one cubic foot of gas, we'd all be running electric cars instead.
Air inside a turbo abides by the same rules. While volume is constant, the temperature, pressure and mass flow all change. If you look closely at any compressor map, it shows these relationships exactly. The vertical axis is for different pressure ratios (or boost), while the horizontal axis is for mass flow rate. At a given pressure ratio, the mass flow rate can be all over the map. Obviously, we want the compressor to be at optimum efficiency, so if you ran across a pressure ratio line and found where it intersects the highest efficiency island, you get the mass flow rate. Comparing between two extremes, at 14.7psi (or 1.0 bar of boost, which corresponds to a pressure ratio of 2.0) a dinky GT12 turbo moves about 14 times less air than a GT60 turbo. That's why it doesn't help to talk in terms of boost.
Instead of talking about how much boost on what turbo kit, why don't we all do our homework and say how much air is moving through the engine? You don't even have to look at a compressor map. Just hook an OBDII scan tool to your ECU and have someone read the real-time signal coming out of the MAF as you tear down the street. The magic number is 10bhp for every pound per minute of airflow. It's a ballpark and will vary from 9.5 to 10, depending on how knock-tolerant the engine is, then everyone will know how much power potential that wheelbarrow-load of money poured into their car has bought. Whether or not your car makes that power is between you and your tuner.