Best V2 performance is found at Vimd (minimum drag) which allows max excess thrust for steepest climb angle.
V2 is not normally the BEST speed, but more of a minimum speed to meet required gradients.
If you have a simple "total drag curve" you'll see that with an increase in weight, the graph moves up and to the right, resulting in Vimd occuring at a higher speed.
Large swept wing aircraft have lower coefficients of lift at low speed and therefore have to accelerate to higher speeds for a given takeoff performance. Sometimes runways don't allow the time/room to accelerate to these most favourable speeds, so a slower compromise is made.
If however, you have excess runway, then you can hold the aircraft down, accelerate longer and rotate at a higher speed, allowing a steeping climb angle and therefore increased takeoff performance. This is called V2 overspeed.