Well done on having plans and options, Shari. My advice for whatever you do, is to get into the best Uni or place like RADA that you can......that was what I was told at your age and it worked reasonably well. Folk wanted me to do Maths - even though it came reasonably easy, I didn't get any "kick" from it, and ended up doing Geog, Geol, Chemistry and Biology at A - looking back, I could have easily done English, Geog, Geol and possibly Chemistry, History (though the O-level teaching was awful), and Art. In English, I surprised myself and my teachers by landing grade ones in both Lang and Lit.
The point of all this waffle is follow your heart and what you feel is your excellence. Belief in yourself (realistically) is just as valuable as the brains!
I agree with you, and was going to question you about science subjects and practicals, though I must say that I am appalled at the lack of practicals, or rather the simplicity of what they do at As....
2sp, I think one major conclusion is that it very much depends on the character of the candidate and their own 'drive.'