Personally, there's hardly any year 11 content that I remember, or even apply in year 12. The more you progress through the course, the more "different" concepts and topics will get (compared to yr11), and as sida1049 said, it is also more focused on current trends in australia and the global economy.
In saying that it'd definitely help if you just skimmed over the VERY basics i.e. supply and demand, opportunity cost, factor and product markets etc. But I reckon the main thing you should do is go ahead with the earlier chapters, they really aren't that hard (Free Trade, economic development, globalisation) - first topic of the syllabus.
Don't worry at all about your performance in yr11. As long as you are consistent in yr12, these gaps in knowledge won't even matter. Key word - consistent.