I adjusted my ALDA the other week and it hasn't affected fuel mileage significantly.
In my situation, it results in less time spent with the accelerator on the floor trying to get the car up to speed and dumping in fuel (concequently having to let off every time you hit boost).
Mileage was hovering around 25-27 and is about the same now...
I have the 617 but still should be similar results.