It really has nothing to do with imperial or metric. The point is do you really want to know how far you can go on a set amount of fuel or do you want to know how much fuel you need to cover a set distance. Km/litre is just as bad as mpg.
The problem with dealing with percentages is that a high figure for mpg really means a small amount of fuel used, so the same percentage change would mean a smaller saving in fuel than a low mpg figure with the same percentage change.
A 100% improvement on 28mpg will save you about 5 litres every 100 km or about 1000 litres a year.
A 100% improvement on 56 mpg will save about 2.5 litres every 100 km or about 500 litres a year. They are both 100% improvements, but one saves twice as much as the other.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.