Your boat/motor, gas, battery, etc. without you in the boat, comes to about 1600 lbs. / 725 kg.
I had a V156 with a 60 HP Johnson, gas, battery, etc. without me in the boat it came to about 1000 lbs. / 450 kg.
I was getting about 40 MPH/GPS / 64 kh.
On another boat if I added a 200 lbs. / 90 kg. passenger, I would lose up to 3 MPH / 5 kh.
You could be right about newer motor size, as the newest motor I owned was from the late 1980's.
I know Merc makes motor that are "High thrust" for pontoon's and heavy slow boats, maybe with larger lower units and lower gearing to swing larger props.
Are you still having proposing problem ?