I have a 2.85m Zodiac and initially had a 6hp Mariner four stroke.
The boat would plane with two on board, achieving about 14mph.
However it took a fair few seconds to get out of the hole.
I changed the engine for a Tohatsu 9.8 and with that on the back the SIB goes like a rocket, planing instantly two up and easily exceeding the maximum hull rating of 17mph.
The Tohatsu 9.8 weighs 26kg, the same as a 4/5/6 hp four stroke.
Contrary to some reports, ours will troll along the canal for hours at just above idle speed without complaining. The power head does shake a bit at these speeds, but as it's rubber mounted it does not transfer to the tiller.
Run at WOT (not on the canal lol...) the engine was doing 5500 rpm. I tried a higher pitch prop and it still planed easily, but took a few seconds to get going.
Maximum speed was similar, around 20mph, fractionally less with two of us on board.
However the engine RPM was about 500 lower for any given speed, so a fair bit more refined. WOT RPM was about 5,000 so maybe the standard prop is better.
I keep changing between the two.