I have had a 150 opti for 3 years/ 375hrs now and have barely had a problem. If I had to buy a new motor I would go and get another opti, no question.
I think you are very optimistic, you would go close to 320km with 200 lts but don't forget you need some reserve as well.
Have a look at this third party test, the boat is lighter that yours, its a river V bottom ally :
http://www.mercurymarine.com/engines...42.43644524429
This one is closer to the mark as far as weight goes but she only has a 150 Opti and its in house test : She gets 1.2 miles per lt but its probably light weighted with minimal fuel and load spinning a 17" Enertia prop, I found this prop to be the most fuel efficient for my boat .
http://www.mercurymarine.com/engines...9.951658066642
And one for a Verado 200 hp :
http://www.mercurymarine.com/engines...42.43644524429
The 4 is my choice, less oil hassels.
there is two types of outboard owners 4 stoke owners and those who wish they did go and count at the ramp saturday and sunday how many large hp optis that should tell the story