I'm doing it!
I haven't completed it yet, but I can tell you my opinions of it. I only did further too and never did methods.
-There's not a lot of questions to practice in the books they provide, and math is one of those subjects where practice is one of the most effective ways for you to be better at it.
-I didn't do this but maybe find some checkpoints books for methods (borrow it off someone or the library, or buy it) and that may help.
-Someone told me the content is pretty much the same to VCE methods.
-I don't think the way they explain things is good at all, but I understand it haha.
-I think it's pretty hard but in my opinion still manageable. I did okay in further with 90 something % usually and with this I get around 75~%.
-I think it's hard to say that if you're good at further will mean that you're going to be good at this because the content is very, very different, even though both are math.
-BUT if you're on the lower side of further though, I personally will highly recommend not doing this.
I think you can also choose an elective at uni to cover up as a methods equivalent as well; many universities have that but you have to contact melb uni if they accept it.