As a general rule, advanced and extension will scale better than standard. But as with anything else, you're better off doing well in a subject that doesn't scale so well than doing badly in a subject that does scale well.
Extension English in year 11 really kind of depends on which module you're doing. But regardless, you'll have to be prepared to do LOTS of reading. And some of your texts will be quite intricate, with gajillions of aspects to look at. And you'll have to write tonnes of essays. It's an incredible amount of work for one unit, but I actually enjoy it more than my advanced class. There's a lot more emphasis on what you can think of for yourself than what you can regurgitate (I think that's partly our teacher though).
Anyway, basically in extension English, you chose a module, and then an elective. All the texts you study will come from the elective, and you'll analyse them in terms of the elective, which can actually be surprisingly broad.
Module A is Genre, and the electives are Revenge Tragedy, Crime Fiction and Speculative Fiction.
Module B is Texts And Ways of Thinking, and the electives are The Individual and Society (this is my class' topic - it's mostly 19th Century England stuff), Postmodernism and Retreat From The Global
Module C is Language and Values, and the electives are Acts Of Reading and Writing, The Language Of Sport and Gendered Language.
The exam is two hours, and it has two questions. One is an essay, with a question that relates specifically to your elective, and the other is a creative writing task which is for everyone doing your module. Some of the essay questions are pretty horrible, but the creative writing generally isn't too bad.
If you've got any questions, PM or email me.
