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.
the MF2 is a decent bit of kit when one uses a 3bar map sensor, the more air being sensed by the map sensor the more fuel gets injected(equally matched), using a yellow,grey,green injector , up to 4 injectors too.
my guess the mf2 can provide at least 10% xtra fuel (using 1 injector)
another benefit is the KE can be kept standard,or just a small amount of increase in current at the EMPR ; so less fuel to be spilled, better economy and performance.