IMHO one thing is the same, with all people after a period of time, driving becomes an auto function. We essentially can do it in our sleep. once it becomes an auto function you need to have expected signals.
Attention grabers are great for taking someone driving in the auto mode to bring the driving function back to primary attention but it takes time, even if it is measured in miliseconds, before they then realize the function. I would rather send the signal to get their foot off the gas and on the brake and then also wake them up.
Again for casual braking waking them up first is fine, my concern here is hard braking where people need to get to their brakes ASAP and miliseconds could become a major difference in the end result....................