The main thing to remember is that 2 unit maths gets much harder from here on in. Well, at my school they decided to leave all the hard topics 'till the end where we were short of time. If you are having problems in easier topics and haven't even began the harder topics, it may be in your best interest to drop to the subject. However, it is noteworthy to mention that your mark of 40% is not directly comparable to anything else for your performance. E.g. 40% at your school may be equal to 60% some where else. If you seem to be at the bottom rank of your class it might make more sense to drop down.
The general rule for subjects that people brand as 'bad scaling' is that if you do excellent in it, they can only take so little possible marks away through scaling. But as with all subjects, if you go bad it reflects in your exam marks, uai etc.