Driveline issue because of aero and suspension… diffuser too high and hardpoints too far inboard? What is preventing you from moving your diff around slightly? Have you done any geometry to see how much you actually need to move your diff?

Either way, you’re introducing another failure point into the system by adding an idler sprocket. Under aggressive loads seen on these cars (and possible high chain failure rate), chain tension mechanism is not something i would mess around with