If you go to Suzuki Marine Website in the USA
and click on any of the engines there is tab at the top of the page which takes you to 'Boat Tests'.
Full test results are given on all the engines on various styles of boats. No RIB's are tested but looking at the fuel consumption figures across all the rev ranges there is not much difference between the boat styles.
At WOT the 115 is averaging 10-11 gph (remember to convert as it is US gallons) = about 38 litres per hour at full throttle. Consumption for all rev settings is there and the speed obtained.
You can also get the same for Mercury if you go to their US Website.