I don't know about the fuel use, but the difference between full boost (1) and minimum boost (7) is about 5 seconds a lap. I'm doing mid-41's at full boost, mid 46's at minimum.

The boost mapping is a throwback to the old days of the series where the drivers had the ability to adjust the amount of boost from the cockpit. These days it really is like a "turbo boost" button but it's limited to X number of times per race.
Since rFactor can only do the mapping that's what we get - but we can only do it in the setup screen, we can't adjust it on the fly in the race.