Jwinder, the counterweight is meant to counterbalance the cartridge on a statically balanced arm such as the RB300. If a heavier counterweight is used, then it will need to be set closer to the pivot than a lighter counterweight would, in order to counterbalance the cartridge. Since the effective mass calculation is based on mass times distance(from the pivot) squared, having a heavier counterweight located closer to the pivot would cause a DECREASE in the effective mass of the tonearm, due to the exponential nature of the equation.
So, if the Heavyweight counterweight is, in fact, heavier than the stock counterweight, then it will decrease the effective mass, compared with the stock counterweight. This will result in a change of the mass/resonance of the system, and also will reduce the moment of inertia of the tonarm and thus will improve speed of warp tracking. Whether this will be better or worse for your particular application will need to be determined by calculation and by listening.