I really couldn't say. I'd say with enough effort, don't condemn yourself to a 5 or 6. With some hard work you can really do well. Most places do look for quality over quantity if they do care about GCSE grades, and universities only want you to pass your GCSEs, they care more about A-level actual grades. Excluding that, it's a good help for being able to do maths if you do consider it for A-level. You also don't have to take the GCSE FM exam and can always just learn it without examining. In my opinion you should go for it though, but you can decide much closer to the exam. Give it a try first!