True, Charlie A had a newish 50hp a few years ago on his family4 (engine prob yr 2000 / 2002 I think) and I believe that was just ok and struggled a bit, but now with the 2010(?) 75 it's a beast!!!!
I think too, the old school engines were rated at the flywheel and the newer ones at the prop. So an "old" 50 isn't as powerful as a more modern 50.
You might find too that the bigger engine is more efficient as it isn't driving uphill / struggling to keep the boat on the plane.
With that weight you might want a 2 stroke for more punch to get the boat going if engine is small.
Then we get on to trim tabs, have a google of SmartTabs - a few of us in the club have them and they are ace!
Rob