One can definitely get 20 marks without including historiography. still, historiography is always a plus.
its not very important to know the exact names, its more important to know that there is a historical discussion concerning something.
Example:
1. AJP Taylor claims that Hitler did not want war (ww2), while almost every other historian claim that Hitler was responsible for causing it.
2. Many historians claim that Germany did not cause ww1, but Fischer claims so.
If you are doing the Stalin section in paper 1 it is good to know Low, because almost every cartoon in paper 1 is made by him, and you can see his signature in the corner.
Another way to gain higher marks is to do name-dropping. it is actually possible to make something up, like make up a name, or take an old uncle's name or something, and say "the famous historian blablabla said that...". the examiners have no time to check, especially if you mention one from a non-english country, sweden in my case. maybe one should not use this technique too many times, because examiners could become a bit suspicious if you do it recurrently.
also notable in the markbands is that you need to challenge the question to gain high marks between 16-20. this is probably not appropriate in all questions, but when the question starts with a quote or something, you can talk against it. and when a question asks "to what extent..." one can also challenge it a bit. to challenge questions is maybe easier done in languages, but it works in history too.