Economics in yr 12 is more applied to the "real word" out there in the global economy. You start applying concepts/theory you learnt in yr 11, and its a given that you have your economics skills under your belt by that time. If not, there's still time to fix it up~
As for studying for eco, my teacher has pretty leet notes so i mostly study out of my exercise book. Summarising the text book is *THE* biggest waste of time because it should ONLY be done if you're not strong with the concepts. But if you don't understand concepts overly well, then go right ahead, it's just very time consuming.
I think a possibly worthwhile study technique would be attempting essay questions which would also be very time consuming, but very worthwhile since people rarely get enough essay practise. They are very useful for exercising your knowledge and if you're unsure of anything you can research the topic again for things you've missed out.
As my teacher says, economics essays are for flaunting your knowledge wide and far.