Let's say one turbo is rated at 550 cfm and another is rated at 800 cfm. Would you still get the same amount of air from the larger turbo at a given boost pressure, or would you get more air volume?
That depends on the given boost pressure, and what motor it is on. A motor with higher volumetric efficiency will swallow up all the air the turbo can produce, providing for less resistance, and therefore less boost than a similar motor with lower VE and the same turbo.