Not that efficient because of the following reasons.
1.EOGs are on when the trains are in pitlines. So fuel is saved only during journey but consumed in pitlines.
2. HOG connection is not possible on a rainy day as I have personally observed many trains running with EOG on rainy days.
3. Only with WAP-7 and few WAP-5 HOG is...
more... possible, due to low production of WAP-7 many LHB trains are running with WAP-4.
It would be better if a mechanism of attaching a pantograph on EOG is developed. This mechanism will ensure directly supply of power to EOG and will be weather proof and will not be dependent on a specific class of loco.