The Bosch indeed does the flow at 3 bar and the cosworth is running 3.5 bar fuel pressure. So if you scale the flow rate linearly with the pressure it would do 446 cc/min at 3.5 bar. The boost pressure isn't important as the flowrate depends on the differential pressure over the injector, and that's is kept constant by the fuel pressure regulator. But even the 446 cc/min doesn't give enough to get 350 bhp.

I've also seen a reference to the 803 injector being 523 cc/min instead of 382 cc/min (all at 3 bar). http://www.cosworth.hu/content/haszn.../injectors.htm
If you scale that flowrate up to 3.5 bar it would be 610 cc/min. And when you enter that value in the calculator you get a maximum power of 358 bhp at a 100% duty cycle. I guess that kind of matches with what it does in practice.

So if I now do the same for the Siemens injectors the flowrates at 3.5 bar would be (all at 100% duty cycle):
55s: 570*7/6 = 665 cc/min --> max 390 bhp
72s: 756*7/6 = 882 cc/min --> max 517 bhp
83s: 890*7/7 = 1038 cc/min --> max 608 bhp

Does this sound correct for the different Siemens injectors?