I did hear from Gary at Ballistic Recreations and his 450C (842F) minimum temp is based on tests he personally conducted. His training as an engineer and ability to test for residual hardness led to this minimum temperature to be used with his system.
I think the heat transfer with the salt may be slightly slower than lead, so this contributes to the need for a higher temperature. I will be ordering a kit soon.
As a side note heating with a torch and quenching with water may actually shorten the time at the annealing temperature. Letting the brass cool in still air would allow more time at temperature. The quenching adds nothing to the actual  annealing other than safety so you do not bump into hot brass. It may keep the heat from traveling down the brass also.
We used to anneal 300 series stainless cable fittings at 2000+F and quenching in cold water actually did something there, it prevented grain growth so the fittings could be successfully swaged onto the cables without splitting. It was easy to see if the process worked, if splitting occurred you did not do it right. When done correctly the cable would break before the fittings pulled off in destructive testing.

Quenching with Ballistic Recreations system will stop the heat from traveling, as well as rinse the salt of the cases. I will ask if a tittle detergent in the quench will help with rinsing the salt off.

The correct name does not have "solutions" in the tittle, that is a bullet proof glass company