It will certainly help you stand out from the crowd (slightly).
But, you have to ask this question yourself! For me, it's about giving up time after school, and so, in my opinion I think it's worth it.
Maybe, see how you cope with it at first, then after a while, if you're struggling, you can drop it!
It's all up to you!
Depends, do you enjoy doing maths?
Your going to need some chillaxing time and if it's not what your going to need, then don't do it.
If your not thinking about a top 5 uni then I wouldn't bother with it. Use your frees to maximise your potential in your other subjects with time left over for social stuff.