It won't have much of an effect, unless you want to go from a 45 to a 50. Personally I think it's more important to build a lexicon that pertains to your specific texts, so that you can discussion these texts as fluently and coherently as possible.
In many cases (as I've definitely learnt in the last few years) being too articulate and/or verbose can prove to be a detriment in your writing, as often the assessor won't be able to understand the point you're trying to get across. Literature should, first and foremost, be about expressing yourself as simply as possible. Use the language that is most natural to you. If it really concerns you try writing sample essays or even sample paragraphs and then go through them with a teacher/tutor/peer and try and introduce more complex language.