I have to agree with Reality Check on this one.
Whilst I am doing Further Maths at A Level next year (I'm only doing this out of passion and talent for Maths, and not for University prospects). Unless you're a timemanagement wiz, you may risk lowering your overall spread.
I know people who've gotten into Med Schools in Leeds, Kings College and Imperial with simply Maths, Biology, Chem at A*A*A, with an amazing personal statement and experience to support, wheras people who did 4+ ended up getting AABB or A*ABC because they only took it to bolster their prospects.
In other words, Uni's will prefer a more concentrated, high achieving spread, than a less consistent plentiful one, the old adage, quality over quantity.
But ultimately, like I said, whilst it doesn't eliminate you or decrease your chances of being picked, you must have a really good statement and experience to back, especially if you're doing a competitive field (i.e Medicine, Law)