First of all, having done both GCSE Maths and IB Maths myself, I must say an A at GCSE is often not a good indicator of how you will perform in IB Maths. Unless you have done GCSE Additional Maths, the jump between GCSE (regular) Maths and IB Maths SL is too large to reliably extrapolate your performance.
Secondly, I really must reiterate the fact that Maths Studies will greatly restrict your studies in Physics. There isn't enough calculus in Studies to sufficiently prepare you for the rigour of Physics, and I'd imagine it'd be fairly difficult for you to apply for any Physics degree (certainly in the UK) with just Maths Studies.
With the US, it's an entirely different story. Any IB subject that is not taken at HL is, in my opinion, virtually worthless as they will not get you any credit. What will demonstrate your abilities to US universities is the overall merit of your IB Diploma, rather than the fact that you took IB Maths SL to "challenge yourself." If you are serious about demonstrating your mathematical competence, consider Maths HL or AP Calc or equivalent (but i'm guessing this wasn't your intention).
So, in brief, if you are applying for US unis, the choice between Maths Studies and SL will not matter so much since they do not earn you any credit, and you can still apply as undecided and decide to major in Physics later on in your academic career.
If you are applying for UK unis, then Maths SL will certainly be held in higher esteem than Studies, but will not guarantee acceptance to Physics courses at top unis.