the stock injectors for the US market are the same number and are thus the same flow rate. I thought the M3 injectors were better but after much research I found that they are actualy 17.5lbs/hr at 43.7psi (3 bar)
and the ford mustang injectors are rated at 19lbs/hr at 33.5psi and 20.5 at 3 bar. the reason for using the mustang injector is for the 4 pintle (hole) nozzle which atomizes the fuel better and produces a better burn and thus more torque. The stock injectors are only one pintle and since most of use have cars over 100k, they could use a cleaning any way. Becareful which mustang injectors you buy or look at because there are early ones that dont have a 4 pintle nozzle but the older single pintle.
Now just a footnote think of the difference in dispalcement of the 5.0l mustang and the little 1.8l M42, these things are made to make HP and rev.