Do it!
Man, it's simple: you can't play with it, do whatever it takes to make you feel comfortable with it. If this means to turn it down to 10mm, do it. It will probably bend in time but that's not important. Your game is more important and spending a fortune on another shaft is not what you want. A turning down will cost you probably 40$. Find a cuemaker/repairman who can do this on a pro lathe and not using sandpaper from the beginning to the end and you're good.
Here is a list of guys who worked with my shafts: (never had a problem)
Steve Lomax, Mike Webb (pretty expensive), Steve Therrien, Joe Callaluca and many other.
All the best,