If you took a fully functioning P8/loom/injectors etc all working perfect, remove it and fit an aftermarket ecu it will not feel any different. People only tend to fit aftermarket to replaced failed expensive systems, that it why replace a 30 year old ecu with a 30 yr old ecu or that you want more functions and control over more additional outputs/inputs. E.G drive by wire, safety cut outs, logging etc.