Yes I find as Bengo finds. If filling the water tank brings the uxter plate up to the waterline or slightly above water (even just a little bit, right at the stern), engaging astern results in large volumes of bubbles, gurgling and the impression a lot of slowing down is going on, when in actual fact there is naff-all stopping happening (technical term).
On the other hand if your uxter stays 2" underwater, I doubt you'll notice any reduction in stopping power.