I do physics at Warwick, and have to say further maths is the only A level that I actually found useful in my first year (and will continue to do so in my second). I would recommend anyone going into the sciences to study further maths, at least at AS, provided they are capable. I can't see why you wouldn't be capable, I myself (and my closest friend) got an A in Addiitional Maths and did my GCSE at the normal time and came out with an A*. I'd say you should do it.