OP,
What model SF speakers are you thinking about tinkering with?
I know that SF use parts that some say are below what they should be using, but changing an iron core for an air core probably won’t make much of a noticeable difference. And on another vein, mine have an 8 year warranty on them that I’d hate to muck with. As others have mentioned, the resale value probably won’t go up a penny even though you added a thousand dollars worth of new parts.