It depends. With the modular nature of the course, you could take it in November, which would be before your first AS level modules in the following January. It is only really worthwhile if you were a few marks from obtaining the next grade - in which case, a remark might be a better option, but bear in mind that grades can go down as well as increase, but it is unlikely to vary by any significant degree. There are always exceptions, though. I think you are right to suggest that AS will feed in to the GCSE. Unless you are intending to enter a scientnfic career, the AS level will supersede the GCSE, but I admire your commitment and determination. Good luck in whatever you decide to do.