Forgive me if this is too much information for you....

Stock fuel pump is a risk at 20psi - depends on age and OEM filter cleanliness.
For 20psi you need 160Lph (42 GPH) and the limit of the OEM pump at 3.9bar OEM pressure (in filter) is 165Lphr.

SL55 internal 5bar FPR = negative, there is no FPR in the pump - I think the pressure is controlled by the Pulse Width Controller and a pressure sensor loop attached to drive the pump. But don't quote me on this - others may know more.

Back to the SRT6....
Some have reported success with
A) a simple pump change
B) others go for larger injectors ...or
C) =A+B

PUMP CHANGE
Go for a WalbroGSL392 (255Lphr) = $150 it'll give you 205Lphr or an extra +20% head room flow at stock FPR of 3.9bar.
An SL55 pump will give you 340Lphr or +100% head room at 3.9bar. IMO this is overkill and $$, plus extra wasted amps with a lot of flow just going back to the tank.

For OEM condition you need about 135L/hr from the pump.
As mentioned above for 20psi and WOT you need around 160L/hr (42USGal/hr)
The stock pump can do 165Lphr at the OEM 3.9bar, but drops to 115Lphr at 5.5bar.
The WalbroGSL392(255) does 205 at 3.9bar and 165 at 5.5bar.
I worked this back to 5.5Bar 'cause there is a case for using the stock injectors at 5.5Bar Fuel Pressure (see below)
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INJECTORS
NOW....To work this back to the injectors...
Divide the 160Lphr by 6 injectors and 60minutes per hour, then multiply by 1000cc/Litre and your stacked 20psi case of 160Lphr goes to 444cc/min.
You then need to divide by 80% duty cycle to give you 555cc/min.

The performance of an injector is normally rated at 3bar across the injector from inlet to outlet and rated for a maximum of 80% duty cycle.
In our case, our inlet manifold has back pressure supplied by the supercharger. So you need to work out the differential from Fuel Rail pressure to Manifold Pressure.
I have tried to do this for 4 case scenarios in the attached table and for 2 different fuel rail pressures of OEM and 5.5Bar. But I have subtracted 0.3bar due to the fuel line friction losses form the pump to the inlet manifold. The actual differential across the injector is highlighted in Blue as D

You can see from the table - if you can get your head around it...;
a) the stock injectors duty cycle is 94% even for the 65mm SC case - but the fuel pump should be okay.
b) the stock injectors duty cycle is 104% for the 65/178 20.2psi boost case and the fuel pump is pretty much maxed out (depending on newness and filter cleanliness).
c) The late2006 E55 injectors (Bosch 0280156304, MB 1130780123) will work with the stock fuel pressure but show 89% Duty cycle for the 21.0psi case - that's getting up there.
d) The 550cc/min 52lb injectors (Bosch 0280158117) commonly used will do the business off the stock pressure of 3.9bar (corrected to 3.6bar).
Lastly for the lower 4 cases at 5.5bar fuel rail pressure (corrected to 5.2bar).....
e) You can get away with retaining the stock injectors but you'd need to boost the fuel rail pressure to 5.5bar to get the need flow out of them.

BOTTOM LINE
-Get the WalbroGSL392 & new filter /w internal 3.9FPR
-Get the Bosch 0280158117 injectors
-Get a tune
That's the way I'm heading.....

Note1: Changing injectors does have some complexity with the adaptive nature of the ECU's closed loop tune and a retune would be recommended. Some have reported LTFT's correcting after an injector change (eg: Tunaglove) - but I'm not sure this will help during WOT.

Note2: Woody mentioned a "climbing" pressure regulator which is connected to the manifold pressure. He's made this available through his connections with Needswings and reportedly worthwhile. The technique is employed in the pre2000 SLK230's (here), however in our case, I'm not sure if the continually changing injector output screws with the closed loop ECU tune which is adjusting pulse width - probably not. Woody can comment further on this.

Update 09/20/2016 - I added Bosch 044 pump data to pump chart