I've just had a quick look at various assessment and test papers we did (maths/Eng/VR) and DS was around low-mid 80%'s at Easter time, although not terribly consistent and had some scores just below that. We moved on to GL and Letts exam test papers in the summer and by then he was getting over 90%. Most lost marks were on silly things - simple arithmetic errors or not reading the questions. I can't comment on NVR, as we didn't do those at all.
Aiming to complete the papers in a shorter time is a very good idea, as there is a lot of time pressure in the real exams and children have to work quickly. Exam technique and an ability to work under pressure are very important.
I don't know what scores your DS is getting at the moment, but if you are at all concerned, bear in mind there is still plenty of time for him to improve, so don't worry. We seemed to be unable to get higher scores for a while and then things suddenly took off.