I just installed 12” bars on my ‘19 RG CVO with no extensions. First time I’ve swapped bars myself and it took me a little under 3hrs. I know it is more involved on a batwing with having to loosen and shift the inner fairing. As for extensions, dealers don’t like to change the routing of factory cables, wires. On my ‘17 CVO Limited a couple of years ago the dealer refused to put anything higher than a 10” on my bike without new cables. They said it wasn’t safe and I would have issues down the road. I ended up with Wild One Chubby 10” and really liked them. In that case I guess I let the dealer talk me into something that actually tuned out ok. FYI, check out Wild One Chubby bars. They are very comfortable, really good fit and finish and less expensive than Factory 47 or Yaffe.
Good luck and enjoy your new bars...