What got me thinking about this scenario in the first place was Vmcg and Vmca considerations.
Would a takeoff using the lower speeds for the weight cover the Vmc requirements for a takeoff at the higer power setting of rated thrust ?
Have however since been advised that airport analysis takes into account the possibility that you may elect to go rated thrust at some stage during the takeoff.
In any case a glance at the V1 /mcg tables in the QRH makes it unlikely that it would ever be a limitation.
Thanks for your help.