You literally answered your own question by posting the ripped-off cartridges and wildcats they based their "new and improved" cartridges on, that they all they might have done was shift the shoulder back 0.050", or simply put a 35º shoulder on something that previously had a 27º shoulder. Tiny little improvements... That really did nothing, which have been proven to do virtually nothing as far as performance goes. Changing powders and bullets and primers has shown more performance increase than increasing shoulder angle or moving it backwards. The only improvement that increasing shoulder angle does is prevent
brass growth, increase charge pressure at exit (which improves combustion and burn), and as long as they also improved the wall taper, then the added capacity will improve performance because of your fill ratio %. And I'm a huge Ackley Improved guy, and have built several, and still have them. There are a couple that drastically improved performance over the parent cartridge, but mostly from the implementation of slower-burning magnum rifle powders that weren't available back during P.O. Ackley's time. Back then, stuffing more powder in it was the only option. Same reason that the Weatherby cartridges developed any sort of fame. Small bullet, big fucking cartridge full of old surplus .50 BMG and Howitzer powder leftover from WWII = HUGE improvement in speed over a .473" bolt face counterpart due to sheer volume increase.
The ONLY advantages (when compared with equal external factors (rifle, twist rate, etc...) the 6.5CM has over the .260 Rem is A) cheaper and more plentiful brass options (from better marketing, b/c let's face it, Remington really fucked up with the .260 Rem as far as marketing goes), and B) the shorter OA case length allows you to seat higher BC bullets in magazines with shorter internal OAL.
The rest were all reheated leftover failed wildcats that never really took off, other than in small BR circles, or died-off from piss-poor marketing decisions by companies that never really gave them a chance, and didn't have the rest of the industry's support, because of shitty marketing, poor outdated twist rates, etc... Had the .260 Rem been given the same treatment as the 6.5CM, the CM would never exist. The poor marketing and 9" twist rate really didn't do it any favors when heavy-for caliber high-BC LR bullets came along just a few years before the CM and after the .260 was already dying.