Tino, remember that the head requirement on the Walbro pump will be far less than pressurising a fuel system, as a result your flow will increase significantly.
Go back to first principles and estimate your maximum fuel consumption based on injector duty cycle from your previous tunes and etc and see if you can get a head vs flow chart for your Walbro. You do not want to be needlessly generating heat with additional pumps....
Not sure if its worth considering a pressure sensor in the line so that if fuel pressure is not above a predetermined limit Motec won't let you mash the go peddle?
Al