Aerodynamics of carrying water

Since in theory the point of carrying ballast is to
improve overall speed....I am curious as to the actual
performance improvement. The shift of the polar with
added ballast is rather straightforward, but not the
reduction in climb rate. Assuming something like
a 25% time of flight in climbing mode, my back of the
envelope calculations for a fully loaded modern ship
in strong condtions would gain somewhere around 7-8%
overall task speed improvement? Anyone smarter then
I care to refine this number?