The open slot on the 915 and closed slot on the 917 will definitely sound different. There are other differences that may impact trying to fit a 915 into a 917 fitting. There's no better guess on using/finding a 915 than trying to match the specs to your fitting. If that shaft was an option in the 915. Or, you could wait for around September. The 917s may have a price drop with the next (919?) release traditionally due in October.

Finding a new 915 Fwy is going to be difficult since they are no longer in production. You would have to get very lucky searching all of the various golf retailer websites to find a 16.5* 915. Your best option is to go ahead and order the 917 F2 with the specs you list.

I also got fitted for a neutral setting and still changed to the 917. I love absolutely love my F2 but I think it’s more down to personal preference than notable performance benefits. I hope this helps

I'd suggest you get fitted even if you're able to find the 15 models.