Just upgrading the ground wire alone is useless. There is more to the charging system of every vehicle then just the ground. If you are going to upgrade your ground wire you also then need to upgrade the other two pieces of the cycle. Alternator to positive on the battery and the chassis to the engine. Replacing all of these with a larger grade cable will let your alternator work to its full potential easier and more efficiently.
With that being said that could possibly be justified as saving gas and very little at that. But in regards to the "saving of power" I can not make any correlations as to how that would be benieficial.