As a mod on its own, it would make no difference whatsoever.
The car would run rich, reducing performance, however the ECU would soon work that out and pull the fuelling back to compensate, putting you back to where you were when you started.
The other issue is that it would put far more load on the fuel pump, because not only is the pump then running at a baseline of 5bar, but when your then running 1 bar of boost the pressure rises to 6 bar, which is seriously thin ground for a standard fuel pump.
The only time you need to touch the fuel pressure, is when your running out of injector duty cycle. Plus with the B5's already stupid 4 bar base pressure, i'd be looking at swapping back to a 3 bar regulator and fitting bigger injectors if i needed more fuel.