Just do it.
If you don't like it your sixth form will probably let you change in the first term.
iGCSES are harder than normal GCSEs are they not? Honestly it depends on what you want to do at university - if you want to do computer science universities love you having further maths and you should definitely give it a try. Otherwise I would only recommend you do it if you enjoy maths and it's your favourite subject - if you're talented at it then you will have to do an average amount of work but if you're not talented you will definitely have to do a lot of work.
If you do go for it but don't believe you are overly talented I would recommend starting off the first half term with an additional subject that you could drop further maths for as you need at least 3 a levels to go to university if you find further maths too difficult - otherwise you can then drop that subject at half term. (I'm doing maths, FM, physics and economics and honestly wish I'd dropped physics in my first year as it's my worst subject by far.)
I would say that if you are the kind of mathematician who ‘ gets it’ then have a go. Sometimes it takes time to stop making silly mistakes and hence losing marks and possibly grades and hopefully this will happen during 6th form. You can always drop to AS further maths if it gets too much (Assume you are starting 4 a levels). On the other hand, if at gcse/igcse you were getting high marks because of constant repetition/practice of papers rather than ‘getting it’ then be cautious with FM as you might struggle with the way questions are asked.
GCSE isn’t always the best indicator. If you really like Maths and want to pursue it further, then go ahead. You’ll find out during the course how well you will end up doing.
In my opinion, Further Maths is marginally more difficult than Maths. Once you get a grasp of the more difficult concepts, it shouldn't be too bad. If you can get an A*/A in Maths, then I'd say you have a reasonable shot at getting an A or B for Further Maths. It's hard to say though because it really depends on the person, so maybe it could be worth briefly looking through the content of each subject to see how well you think you'd be suited to them?
I have an A* (with 95%) for Maths but it's spec A and A*-G syllabus (I don't really know the difference between specs but apparently spec A is a tad bit easier than spec B). Do you think I'll be able to do Further Maths? It seems a bit hard and the new syllabus is supposed to be harder. However I also really like Maths...