It seems that a lot of people are ignoring the effect of the pressure drop from turbo to intake manifold. This value plays a HUGE role in how much power/boost/flow these turbos can provide. I'll run through an example to show how important this data can be.
First, we need to state our 'conditions', and make some estimates of the N54:
sample RPM: 6500rpm
VE @ sample RPM: 90%
stock boost @ the intake manifold: 8psi (ie, PR = pressure ratio = 1.55)
N54: 3.0L = ~182cu. in.
stock IC pressure drop @ 825kg/hr airflow = ~2psi (this spec is according to BMW; 825kg/hr = ~412cfm)
With the above sample points, the N54 needs 478cfm in total @ 8psi intake manifold pressure and 6500rpm. Each turbo would thus need to provide 239cfm (or .113 m^3/sec). This .113m^3/sec value gives us our x-axis lookup on the compressor map.
Now, with 478cfm required @ 6500rpm, the pressure drop from turbo to intake will be slightly higher than 2psi. Theoretically, it would be (478/412)^2 * 2psi = 2.7psi. Thus, if we have 8psi in the intake manifold, the pressure right after the TURBO will be 10.7psi (= 8 + 2.7), for a PR of 1.74. This now gives us the y-axis lookup value for our compressor map.
So, with a PR of 1.74, and a flow of .113 m^3/s, we see that the stock turbo has an efficiency of ~67% (which is not the greatest).
Next, let's do the example of 13psi of boost in the intake manifold. Our new numbers would be:
3.0L @ 6500rpm w/13psi = 584cfm (ie, .138m^3/s per turbo)
Pressure drop through stock IC = ~3.8psi
Pressure at turbo outlet = 16.8psi (ie, PR = 2.16)
So with .138m^3/s and PR=2.16, the turbo is operating with an efficiency of <60% (basically, not even on the chart, but close enough to estimate).
Now, let's take the example of 17psi of boost in the intake manifold. Our new numbers would be:
3.0L @ 6500rpm w/17psi = 669cfm (ie, .16m^3/s per turbo)
Pressure drop through intercooler with 669cfm = ~5.3psi.
Pressure at turbo outlet therefore = ~22.3psi (ie, PR = 2.54).
So with .16m^3/s of airflow, and a PR of 2.54, the turbo is operating OFF THE CHART. It's not even rated for that flow and pressure.
Hopefully, these calculations can give people an idea of what's reasonable to expect from these stock turbos, and what scenarios are just looking for trouble.