Unfortuatly most people will never see the increase because alcohol injection always leads to the lead foot.

People tend to drive harder when they have more HP. At low speeds when not boosted your not going to see any gains with it.
When your into the boost at your start point and accelerating you will be cause it takes less throttle for the same level of hp you would be making at a higher throttle position. say X amount of hp at 45% TPS with the alcohol off vs X amount of HP at 45% TPS with it on.
Most people are pretty throttle happy so the economy stays about the same, TD truck owners are the ones that normaly get the MPG increase because they tend not be that way especially when towing.