I'd say do as well as you can this year, and if you get an A*, only then consider it.
Once you start, it's not that bad, it's just the amount of stuff you'll be going through so quickly that you'll have to keep up with. One of the pros is that when you do the harder stuff in AS like C3 and M2 as a further mathmatician, it makes modules like C1, C2 and M1 look like a joke. So you'll always be ahead of the curve. You want to make sure that you actually understand the maths rather than just learning the formulas and the procedures, because when it makes sense to you, it's just so much easier to understand and apply to questions.
Finally, start looking at the entry requirements of the universities you want to go to, because as someone else said, I think A2 further maths would be a bit too much. So another option would be to do normal maths for the first year, and then if you're results are good then you can do AS further maths the second year. Although it isn't a full A level, it gives you a flavour of further maths and it's a good thing to talk about in your personal statement, showing your initiative and stuff