I've had conversations with a few of the slug makers, including the schrifle. Once again, no free lunch. You can only do so much with that low a pressure.
I shoot mainly 20ga, and best slug I have found to date is the Lightfield 3". It stays supersonic up to 100 yds and performs well although if it hits bone it self destructs. Great accuracy though.
The shop ordered in a few cases of the Remingtons and my total experience is 2 deer. One quartering shot was interesting and found bone from the near side on the far side after field dressing. Very accurate at 100yds but haven't tested them further at this point.
Remington must have consistent sabots because accuracy rests on that.
One of the makers told me that differential cooling/heating in the injection molds is the main issue.
I may go old school and try paper patching some shotgun slugs this year. I would rather have accuracy and mass over a flat trajectory.
Maybe I'll come to my senses and just switch to muzzleloader full time.

Tom