So your fuel pressure with half a bar or so of vacuum is 2.8 bar, and with 1.1 bar of boost is 3.5 bar?

If so, I would say that you have a problem with your pump or pump wiring, or possibly even the diaphram in your regulator, as if its 2.8 on idle, it should be 4.5 or so when running 16psi of boost!