IIRC on the B747-400 the first push of the TO/GA switches gave you a 2000fpm climb and the second push gave you rated go-around thrust.
On the B767 you get thrust appropriate to give you a 2000fpm climb and if you want more thrust you have to apply it manually.
So it shouldn't make a lot of difference whether the aeroplane is heavy or light.
Regards,
BH.