5th injector isn't really needed at that state of tune. There are plenty of people over here running 20psi without using one.

I beleive 14.7psi is equal to 1bar.

You don't need a new ECU to run more boost but you will need your ECU to be chipped or the cars overboost protection will kick in.

The oil breather won't do any harm and will help the engine to breath as it should.

The fueling on these cars is governed mechanically rather than electronicly. You just need to take care in setting the car up.