Like jay mentioned, there are some abilities that gain no real benefits from raising above 5. What that list is, hasn't been made yet.
To answer your Sneaking question, yes. The cones of sight get smaller, but it's entirely unnecessary, because 1: the cone of sight is already ridiculously tiny, and 2: Sneak already costs only 1 AP, which is honestly the only reason why you'd spend 5 points into it anyway.