My multiple choice is never any lower than 16, but I can never get 20/20, it's always in the 18-19 range.
They recommend 40 minutes, but I usually spend 15-20 minutes working on it, so a good minute on the 'hard' ones. I always make up time with the easier ones, where once I've read the question, I know what the answer is, before looking at the answers (when you read the question, you gather ideas/thoughts and you can work out the correct answer before looking at all the possibilities).
I don't think 5 minutes will guarantee 15+,
In 5 minutes, you would probably get 5-7 marks, at that.
If you want to improve your marks in the multiple choice, from a 5-7 to about 14, 15 or even 18+, then you should really be concentrating on the wording, looking at all the answers, instead of "guessing" or "it's always C" -- then systematically remove 1 or 2 answers, and then having to choose between the 2 equally correct answers, with 1 "more correct" than the first.
I would recommend you spend 20 minutes if you're struggling with the multiple choice, that's half the allocated time slot, but if you would prefer to make up your marks elsewhere, say in the short answer or option topics, then go for it.
1 mark in the multiple choice could be made up for in the written section, when you write 1 definition or 1 sentence, so make the most of your time.
So, you should spend more time on it, if you're going to rush through it, double the time that you'd normally spend on it, from 5 minutes to 10 minutes, and if your marks double, then spend a little longer.
The most I would ever spend is 20 minutes on multiple choice, for 20 marks, I would normally aim for 12-15 minutes tops, before I move onto Questions 21-24 (short answer/scenario).