It is a heck of a lot of work.
English Extension 1 is already like having an extra 2 unit subject, although it only counts as a 1 unit subject.
Don't do it because you're acing advanced, but rather, do it if you are willing to make your own sustained composition over 6 months, research, document, draft, re-draft, have it torn to shreds, edit, re-draft again..
.. until it's perfect for marking.
In other words, it is a painful process, and should only be undertaken if you actually want to write about something that is completely non guided i.e. make a statement. Then, it would be highly rewarding.