Yes the more boost, the more fuel you will use, however as said under normal driving you won't make no boost anyway, it's only under acceleration/low gears or high speed you actually make higher boost.

All the "Hi-Low" boost switch does when open is cut power to the amal valve and so you run lower boost, when the switch is closed you will be able to run the maximum boost your setup permits.

Basically as said the best way to save fuel as with any engine don't accelerate hard and don't hold onto lower gears longer than necessary or go high speed.

You need to be careful with boost controllers you don't adjust the boost to high otherwise you will have a meltdown.

The ECU fuels accordingly for the amount of air present at any given time so no it wouldn't overfuel.

When you start to increase airflow etc (I.E with a higher boost pressure) the ECU cannot fuel for this change as it is not calibrated for it hence it would run lean, this is why you need a remap/chip that has been calibrated to the spec along with different ignition curves etc.

Also good mapping will give better fuel economy.

Martin