But what about the dimensions of the cut?
Our boat can hit 9 knots or more on a wide deep river, but no more than the statutory 4mph, often 3 or less, flat out on a shallow/narrow cut. I won't get all geeky and technical about fluid dynamics - but just look at your prop pulling the water fast past your hull when you are going too fast on a shallow silted up cut and you'll understand why there's no point trying to open up the engine in such circumstances.
I drive an old Town class with a Lister air cooled - which is noisy on tickover so the moored moaners think I am going fast when I am not.
The earlier contributor who pointed out that it is speed through the water and not engine speed that affects the bank/moored boats etc. Our boat is so heavy she is like a tanker - takes 100 metres to slow right down from cruising speed. As I say to our trainee skippers - ANTICIPATE, ANTICIPATE!!