Think about the mass of liquid propane injected in ratio to the air mass, I can't see any way that amount of liqid propane expanding will create a 3-5 psi boost effect, should do the math on it I suppose.

The newer Schwan's food delivery trucks have multipoint sequential liquid propane injection, that technology is owned by a company that is owned by Schwan's. On the engine it looks very very similar to a factory gasoline EFI system.

http://www.bi-phase.com/index.html