Well, I did my IGCSE Maths last year 'cause I'm in the top set and most of the other people are doing them this year. The IGCSEs aren't as difficult as you think, to be honest. It's not hard to understand at all. Everybody in my class got an A* last year and we're doing three AS modules this year (C1, C2, S1). This is because we've basically done C1 last year before and after the IGCSE exam.
To begin last year, the top set had 18 people. We basically used one year to go through the whole course. After the mock exams in January, three people were dropped down to set two 'cause the teacher felt that they didn't really understand a lot. So what I'm guessing is that if you do your mock exams and you don't have good results, the teachers will deal with it. Just stick around in the top set for now and try to absorb as much as you can. After all, if you understand, you'll be able to do the exams this year, and if not, you can re-learn it so that it's easier to understand if/when you drop to set two. Or, alternatively, you can ask your teacher during break times about things you don't understand. I'm sure they'd be delighted to help. And if they find that you're really hopeless at this stage, they'll take action.
However, if you really don't understand anything, then I suggest you make a formal request to be placed back into set two. Since set two is slower, it will give you more time to understand and to digest the topic.
Another thing is that if set one is taking IGCSEs this year and set two next year, you're going to have to think about the AS-Level. I would assume that set one is also going to do AS-Level modules next year, whilst everybody else is doing IGCSEs. AS-Level is a huge jump from IGCSE. The stuff is a lot harder to understand, especially if you're not naturally gifted in Mathematics. Have a talk with your teacher about the AS-Level. He/She should know whether you can handle the AS-Level modules next year or not. Then make your decision, or have your teacher do it for you.
Hope that helped in some way.