My friend works 12 hours a week and achieved AAC, whilst I work 9 hours a week and achieved A*BB. We both got into our first choice universities, but even through only working 9 hours a week, I was shattered. It's fine when you don't have any other commitments, but consider this:
Any free time, you'll have will be devoted to working or doing schoolwork. You need some time to yourself to rest, otherwise you will definitely burn out. My school advised doing no more than 10 hours per week, so I'd suggest going down to that at a maximum.
15-25 hours is probably more than you think.