Really depends on the person. I got A* in gcse maths, then did all of A levels maths in one year and came out with an A* without too much effort. However I have a friend who got an A in gcse, did A level all in one year like me and got a D so is resitting a lot of stuff. I have another friend who got a B at gcse, did as maths last year and got a U and she (seems to) spend loads of time revising.
I think A level really test your ability to actual understand a subject rather than just remember stuff or be good at answering exam questions. Like i got A* in gcse physics but then only a B in as physics and I've come to realise I was only good at gcse cos i was good at answering the exam questions and I don't actually understand physics that well. So I think you need to analyse why you're getting the grades you're getting and if it's cos you actually understand the subject before you choose to do it at A level.