A EFI setup can take different injectors and be remapped pretty effectively so its fair to say that we are talking about a MFI based engine here when talking about 5th injectors.
The problem with MFI even with a 5th injector is the MAP. Having only 1 bar Map sensor built in the ECU will mean there is a set Map upto 14.7psi (1 bar) running anymore boost than that will create problems after a period of time and damage your engine because the ECU doesnt know what to do after that point and just throws in aload of fuel with is always uneven in each cylinder. (Port 1 and 4 have less air and fuel due to the shape of the inlet manifold)

if you have an MFi system running on the limit of the management + 1 injector (Which cant supply equal amount of fuel to each port anyway) how long till it damages the engine due to the setup being out (just slightly but at high boost thats all it takes)

But then again like you say its Complete bollocks, there's a lot of high powered RST's running 5th injectors around as evidence of this

To spend the money on a ZVH to then skimp on the managment by running a 5th injector is not a good idea imho... This is why I stated if you have fueling problems look for better management.