Honestly, I don't think the GCSE itself would make much of an impact really for anything, and I would say just concentrate on that A* in maths. I assume you're taking it year ten, so does that mean you do the ad maths course in year 11? If so, that would be very good as it of course shows a pretty natural aptitude for the subject. However by the sounds of things you'll be taking stats outside of school; I think this would look quite good to be honest as it clearly shows your interest in the subject as you're learning about it outside of lessons. Personally, I would only take it if I was certain I could get an A* in maths, as that should be your priority. But if you feel you can definitely do it... Wouldn't hurt I guess