What does this mean that the RACE 5% OVERDRIVE HARMONIC BALANCER TO SUIT HOLDEN ECOTEC L36 L67 S/C 3.8L V6 will need to be mirror balanced to the current Ecotec balancer for correct installation?

The race balancer is balanced to suit the L67, not the Ecotec. Therefore, prior to fitment, you'll need to get it mirror balanced to that of an Ecotec balancer. Secondly, as this balancer was designed for the L67, there will be a groove that you won't use as it is meant for the supercharger setup.