Why did the factory use vacuum-referenced fuel pressure instead of a fixed pressure, and would a fixed pressure make the ECM easier to tune?

Why reference it: an injector's effective flow depends on the pressure difference across it, which changes with both rail pressure and intake vacuum (LSX Magazine). A regulator referenced to manifold pressure raises and lowers rail pressure 1:1 with it, so the difference — and therefore injector flow — stays constant whatever the manifold pressure. It also stops high vacuum from pulling extra fuel through the injector.

Would a fixed pressure be easier? Not necessarily. With a fixed rail pressure, the pressure difference changes with manifold pressure, so injector flow and dead time change too. HP Academy says the tune's fuel-pressure reference setting (atmospheric vs manifold-referenced) must match the hardware for exactly that reason. Either system works if the calibration is set to match it. See fuel, E85 & boost.

Want to do it yourself? See who tunes it, and how to learn.