I normally say that WT+ is worth it, but SYD is one route where I seldom think it is. The price is usually double or at least £1,000 more than the WT price, as it is in your case. I would not consider that worth it, especially when you consider what you could spend £2,000 on in Australia. If you've already booked WT, I would stick with it and make sure you have quality accomodation, food and travel arrangements when you get there.