I did a bit of a study on this for months.
On the back of the Scorpion 7,5 I had an Evinrude 150 Ficht. At 3750 rpm it did 50km/hr with 32 liter/hour. At 5000 rpm, 60km/hr, 53l/hr.
Later I changed to an Evinrude 250 DI:
At 3500rpm also 50km/hr with 32 l/hr.
At 4000rpm, 60km/hr, 40l/hr.
As you can see the 150 was too light for this boat. With the 250 at 60km/hr it's 13 liters per hour cheaper. At WOT it is not
85 l/hr, but with 98km/hr
I'm very happy with the change. Especially in the rough it takes less fuel to get over the waves or against the tide.