I don't know precisely what happens with a little added resistance in the injector circuit, I assume the injectors are slower opening, which might cause some leaning out at high speed. It would be interesting to see dyno results but I suspect the effect is very small. I generally use 4mm twin core cable (one core each side) to the relay which is mounted close by. Remember also the return path for the injectors is thru quite small yellow wires so I suspect the feed wire size used is simply "traditional".