I agree that future bullying is not something to base the decision on.
I think I would find this an incredibly difficult decision and would be torn.
I think I'd consider what my wider family, and DH's wider family are/were like. I think a 24 digit child would be welcome and although there might be one or two relatives who would counsel to have the operation(s), most would be thinking of ways to help the child feel welcome and accepted, without the difference completely defining them. We don't have any olympians or musicians in the family.
I remember a girl in my reception class who had an extra digit growing out of one hand sideways. She couldn't move this sideways growing finger. She had two fingers and a thumb on the other hand, and a nubbin with no feeling. Her parents had left the decision of what to do in the hope that they would be useful to her, but at age 5/6 they all decided to operate to have the sideways growing digit removed. So the operation can be done at that kind of age, if the child really wants it.
You can move from schools where there is bullying, but most schools will have other children with all kinds of differences and should have a culture of acceptance and inclusion.
Teasing is a part of most childrens lives...I'd rather my child learned how to accept it and deal with it rather than do it! The thing about making these changes is how does it then affect how the child relates to others? Will it make them feel inferior or superior to have had this done to them?
I think I would also want to meet (or at least talk online to) others who have chosen to not remove the digits.
I'm still undecided but I am swayed more by the arguments for keeping the digits, over the ones for removal.
If it were a cochlear implant, I'd be against getting one for my child (DH would want one for a deaf child - good job we don't have a deaf child). This is because I think the risk far outweighs the gains. Nothing to do with bullying; everything to do with operating on the brain and the chance it might not work. I don't judge others who have CIs, nor would I try to persuade someone out of it, but the reality is an extra thing for a child to have to deal with, many appointments through life and replacement operations every 15 years or so,, and it doesn't actually completely fix anything.
If it were anything cosmetic that is the 'normal' thing to do, I'd go with it and get it done (braces or veneers for teeth; lazer treatment for wine stain birth mark, etc.) Because I don't see those as a part of the child's identity and there isn't the same level of risk as, say, a CI.
Equally I'd want any procedure offered which would greatly benefit the long term health of my child. I'd never refuse something like a blood transfusion, or an operation to enable them to use their existing fingers.
I don't think you can make a wrong choice here. There are strong arguments on both sides of the debate. The most important thing is that you do whatever you do out of love for your child and that is what he growns up knowing (and that the other choice is also a loving decision for many people too, clearly!)