I did the same as what you're thinking about. I personally enjoyed it very much, but at the same time I also thoroughly enjoy maths. If you're super in to maths, just go right ahead. Something else worth considering might be a uni extention program, but generally (or always? I'm not exactly knowledgible in the area) you would've completed methods in year 11 instead. The only problem you're really going to run in to is being bored (or more accurately, having more free time

) - there's a lot of overlap in methods and specialist and I found that aside from getting ahead or helping my friends in methods, there wasn't much to do. Keeping two bound references is also a bit tedious, but certainly not something that should influence your subject choices. I put two lunch times into it a week and I was fine.
Most people complete methods and specialist in the same year. I think further maths is a very practical class and I'm glad I did it. There were times that I used something I learned in further maths in methods or specialist. If you're not worried about having one of your maths forced into your bottom two, and there's not another class you'd rather be doing, there's no reason not to. Honestly it didn't change my ATAR (although I was worried about it); Further Maths ended up in my bottom two simply because it was one of my lower scores.
I'd recommend it if you like maths and you're not concerned about the bottom two thing. It's reasonably common.
