RR- I don't shoot competitively, haven't shot past 600 yards, and have no experience with annealing.
That being said, I do recognize the significance of consistent neck tension and its influence on accuracy. Reducing Grendel neck diameter 0.010"+ consistently in one step is a lot to ask. My approach is to use a standard full-length resizing die (Forster), followed by a
Redding Competition Neck Bushing Die in 6mm PPC. This method would allow you to leave your current sizing die as is, and use the neck die for the finishing touch. I've used both Wilson and Redding bushings (Redding is undersized more so than Wilson), and prefer the Wilson since they seem to have a tad of flare at the bottom, avoiding the potential of a ring near the base of the neck. Also, I often see accuracy enhancements using a very light crimp with the Lee FCD.
Again, great post to open this thread. I might have to follow in your footsteps one of these days and visit the Butner facility.