The FMU basicly acts like a plug.

Upon sence of boost pressure from your intake manifold, it plugs the return so that your pump is still pumping fuel into the rail but the excess can't return to the tank so it builds up pressure so that when your injectors need it.. it's there

You will force a little more fuel through your injectors like this but your should definatly upgrad your injectors so that they can dump more fuel at each injection.

hope that info helps ya out some