A narrower stance width may be marginally more aerodynamic, but often is unsustainable to give a performance benefit. Perhaps it could be beneficial in short track events such as the pursuit, but again will largely depend on individual factors.
Ultimately, a stance width that allows you to apply power directly down through your driving leg will generally be more efficient and comfortable, allowing you to continue to cycle without pain or injury for longer, allowing you to get faster!
Stance width should be adjusted to what suits your biomechanical and anatomical needs. which will optimise comfort, performance, and prevent injury. The aim is to achieve good hip, knee and foot alignment by aligning the foot to the knee. Since Q Factor is largely determined by the bike and components, this is done by making adjustments to the pedals and cleats, as well as ensuring the overall bike fit is appropriate.