The only benefit in doing 7 subjects is that you might improve one of your bottom 2 for 10% of it's worth. In your scenario, given your SS estimates/hopes, this would involve jumping a 4.2 up a couple of decimals at most. But as I said before, this comes at the expense of all your primary four for 100% of their worth which could involve many more aggregate points. And since you don't seem to value Maths as your strongest subjects given your estimates on Specialist/Physics, and your existing Methods score relative to your others, why pick up another Maths? At least do another IT or something that you seem to have a genuine interest and strength in. But ultimately, don't bother IMO.