This thread and shooting yesterday got me thinking (dangerous i know)
But i started thinking, what if the traditional max speed for cast boolits is due more to this rpm max with high speed twist rates (production rifles intended for jacketed bullets) rather than a true max velocity achieved with a slower twist.
Then i noticed your group was shot with boolits with lube rather than powder coat.
Without having looked into it more, i expect this to explain why some cartridges are more "cast friendly" than others. ... cause production rifles use slower twist rates.