galvanic corrosion in the wild: where is the reduction?
Comparing galvanic corrosion to the zinc/copper battery is a bit unfair. A fast corrosion rate is still much much slower kinetics compared to the expected rates of the electrochemical reaction in the battery. 

The counter reaction in the galvanic corrosion case is almost always reduction of water. This is why it is much worse in conditions near the sea.

Galvanic corrosion rate can be measured and there is an ASTM standard(G71) to make the measurement. 

To answer your comment below, I am trying to say that it is impossible to say if it will double or triple or just increase by 10%. The factors are complicated and hard to predict a priori. That is why it is a good idea to measure using the above mentioned standard.