Yeah the other two guys are entirely correct. Just do as well as you can on your SACs. Let the Ranking of yourself in your cohort worry about itself.
I also did Further last year in Year 11, and I was not even at the top of our cohort. I would have been Top 10, but that's about it. What really matters, is how well you perform in the Exam as well. There were people in our school who got 100%'s on SACs, but when it came to the Exam, they got cocky, and didn't actually study. They didn't even end up getting 40s, and in fact, no one in our 2012 cohort got a 40 for Further.
Use this as motivation, to do even better than what you are getting now.
It's GREAT that you are doing a Unit 3/4 in Year 11. Do as well as you can, so that it's a bit easier on you next you, but don't get disheartened. Take it as a learning experience for when you do Year 12. I certainly learned a lot from doing Further last year, and it has helped me out so much this Year.
Good Luck bud!