Why do you think practicing harder NVR papers would help ?
From the sample papers that schools publish, the nvr component does not seem difficult-the difficulty is in the number of questions that are required to be answered in a short time.I would focus on technique and speed on a variety of papers.
Have you had experience of an actual test? I would be wary of taking the sample paper examples as the level of complexity required... but do agree the timing is also a very big issue. I just want to ensure my dd has seen as wide a sample as possible - if I have found ones that she is good at and someone says that another publisher's are a lot lower standard of complexity, I will place less emphasis on those in our practice sessions - we are entering for CEM and the normal Essex CSSE exams and therefore are tight on time for nvr as it isn't in the standard Essex CSSE.
Does that make more sense now?