thisThis can help you a little bit, I hope.
Its about Examining the politics behind Ferrari's willingness to supply Red Bull instead of Renault in 2016 after Mercedes went cold.
http://www.skysports.com/f1/news/12475/9995927/f1s-engine-politics-examined
I guess even with the same engine, there is lot more work happening around the car like the design and aerodynamics (the creation of downforce to help push the car's tyres onto the track and improve cornering forces and minimising the drag that gets caused by turbulence and acts to slow the car down), and I think the original makers will have the perfect idea about the required aspects needed around the engine.
I'm not sure about giving engine that is underperforming to suppliers, as there were some allegations during last season.
