What you get with the Creedmore case design is a longer neck. Where that is valuable is for loading very long (heavy) for caliber bullets. It was designed for long range competition. 143-147 6.5 pills, 105-107 ish pills in 6mm, or there abouts. I doubt there is an animal that can tell which 6mm got em, or which 6.5. The 6.5CM is likely to end the 260 as a factory cartridge. The 6mm Rem has been basically dead for decades.
Myself, not handloading at this time, if I wanted a 6mm bore, I’d buy a 243 Win, if I wanted a 6.5mm, I’d buy the 6.5CM. If the 6mm CM takes hold like the 6.5 did, then it will be viable. The number of 243’s out there suggests to me the 6mm CM is a long shot outside LR competitions. Right now the 6mm CM is too new to call it one way or the other. 10 ish years ago, I bought my youngest son a 260 as his first BG rifle, really wish it was a 6.5 CM now, due to more factory ammo availability...
All that said, my 25-06 is my fav medium game cartridge.