It just depends really, going ahead advantages some people while it may disadvantage others. For me it was best to go ahead of the class (the pace of my class was too slow, so I basically went through the whole year being ahead and learning out of the textbook). as said above though, if you do this you should make sure you consolidate what you know. For me, I made sure I had deeply covered the previous content before moving on. There's no point going ahead if you haven't understood the current content well enough. This is where it can start to disadvantage students. Some go ahead, when they haven't fully grasped the basics/earlier materials and concepts, which leads to more problems down the line, with their learning being all over the place, having to come back and redo topics.
Yes, in the assessment reports the examiners consistently mention that students have proven to be weak at vectors and complex numbers, most likely because they haven't made the effort to go back and review the content. I have some work to do on differential calculus, but these holidays I'm intending to do a thorough review of said topics. 
I think that's more because for a fair part of the state, vectors are new territory (yes you might say that you cover them in GMA, but as I've said previously on here, not all schools cover GMA in the same way). When we started vectors, none of us (besides me and one other kid, because we'd gone ahead on the holidays), had be exposed to vectors from a maths point of view, and others hadn't come across them at all before. Which is why most of the class struggled, it took time to get their head around vectors. Unlike the most of the rest of the course (besides maybe complex numbers), they'd done something similar, just at a lower level before, in methods or previous maths (again, depends on the school, but just remember not all school are up to the same standards in the maths before yr 12).
So in short, if you understand everything that you've covered so far, and have the time to go ahead, then it may be beneficial, but just don't rush through topics for the sake of rushing through them.