Will this fit on the 2015 DDC model (or will it even be necessary)?
Yes it will fit and work on a 2015 model equipped with DDC. As far as being necessary would come down to rider preference and use of the bike.
If you want to have the ability to adjust your rebound and compression individually on the front forks then yes it would be necessary.
If the bike is going to be used at the track then I would say yes 100% it would be necessary on the bike.
Any other questions you may have feel free to email me @ [email protected]
I look forward to hearing from you.