The 5th injector will only provide around 5% extra fueling so will only gain you a couple of PSI.

Stu's injector controlers will control a larger injector like a Cossie Yellow/dark blue or even a green injector. The difference between Stu's controlers and an MF2 is that the MF2 is 3 dimensional and can be set to come on at a certain rpm if the boost is over such a PSI etc when it starts to run lean. I don't know if this is still the case with Stu's controlers of if they have developed them further. They also make good water injection controlers.

My car has been set up to run 14psi but runs lean at 5krpm so can't rev it passed there. The fueling has been wound right up and I either have to up the fuel pressure, get a 5th injector or ditch the whole system for something not prehistoric! KE-Jetronic on my car can only supply fuel for the whole rev range with the boost set at about 11 psi. It just can't cope without more indept modifications.

I advise you get the fueling checked by a tuner before you do anything....