Sounds like a great cartridge. The 6.5 SAUM and the 6.5 SS are both using the SAUM case. The 6.5 SLRM is based off of the RCM case,
I believe the WSM case is just a bit too big to use the 6.5 mm bullets. It works, but not like the 6.5 SAUM does with its nice barrel life, ect. The RCM is a great idea too. It has less capacity than the SAUM but the 6.5 SAUM has some minor issues with being able to fit in a standard short action magazine when the bullets are seated out to the lands. Minor issue and easily fixed. The 6.5 SS is a shortened 6.5 SAUM with a few other tweaks to the case to get capacity back up. It's a true short action cartridge with and improved shoulder and a nice long neck.
Until we see the specs on the 6.5 SLRM we are only speculating. But it sounds like it will be shorter than the SAUM, putting it real close to the 6.5 SS.
I'm a huge fan of the short, fat, efficient cartridges. And most certainly of the 6.5mm. I look forward to another. I think these Short magnum 6.5's are here to stay. And that's s really cool thing.
My hats off to the guys at GA Precision who came out and developed the 6.5 SAUM and made it a popular choice. And now to Gunwerks for taking it a step further, and hopefully not to long from now, will have head stamped brass for us like they did with the 7 LRM.