The best twist also depends on distance. A lot of 6.5 Grendel types will tell your that 1:7.5" will give you the best groups within 300 yards but 1:9 or slower will be better between 600 and 1000 yards but not as good closer. The right tradeoff point is different. At shorter distances, "overstabilized" can be good.
Mike
PS. Based on my own calculations, I am of the opinion that for 5.56/223 1:8" is the best all around twist if you don't need to shoot tracers (1:7" if you need to shoot tracers). The Seal Recon Rifle is 1:8", the Army SDM-R is 1:8", and the Marine SAM-R is 1:7.7" so the guys who shoot on 2-way ranges for a living seem to agree.