Last year my son and I made a trip around england and wales. We ran a Humber Ocean Pro 6.3 with Suzuki DF115. we made 1.15L per Nm. I would not expect any worse with the bigger engine you propose if run sensibly. The smaller engine often has to work quite hard (given the weight of boat and fuel load carried)
We sold the boat to buy bigger and diesel (as we could see that extended cruising with Petrol was problematic!).I hate doing that jerry can bit. Havnt yet purchased boat.
Have a look at our trip
if the link is not correctly formed search on 'father and son'
If you would like crew/company and some percentage shareing of fuel cost would be pleased to meet and discuss further.
pm me to gain email address