Sparky,
The use of a mechanical pump for an EFI turbo is because it has sufficient flow to maintain the required fuel system pressure at high horsepower and boost pressure. If you have a big motor at high boost the fuel pressure has to be set at the control pressure, typically 40-50 psi plus the boost pressure and most of the present day electric fuel pumps can't provide enough flow to be able to maintain the required 60-70 psi fuel pressure. The guys to talk to are the Langlo's, they have the fuel system on their big Hemi pretty well dialed in. Their motor is making about 2500+ hp and using a BSF (Brake Specific Fuel flow) of .6 lb/hp-hr they are burning around 4-5 gallons of gas per minute!! Only mechanical pumps can do this at 70+psi.
Rex