5th injector is a slightly crude way of increasing the fuelling in general. If the standard injectors were near the limit the best thing to do is A. Bigger injectors or B. Dual injectors (2 x small or 1 small & 1 big per cylinder).
Both A and B will need some way of being controlled though i.e aftermarket ECU £££s or remap at minimum.
1 extra injector split between all the cylinder will increase the fueling, but unless the design is spot on then some cylinders will get more/less fuel (so inconsistent across all cylinders).
And manually controlling the injector via a switch