What would be helpful, if you've also thought of it is: is this a skill that you can "switch off" when it's not required? If I were in a setting where it's safe to do so, it could be of value to turn off SA and focus more on - a book perhaps, or my own thoughts.

Based on my experience, yes. I am an absentminded person currently trying to retrain myself to function as a critical care nurse–see my related post here. At work/in clinical, I am slowly developing the skill of being aware of everything as it happens, keeping an ongoing plan/list of priorities, separating out important changes in a patient's condition from noise, and knowing when I have to re-prioritize. Of course, focusing on modelling the world around me constantly (or at least the few cubic meters of my patient and relevant equipment) is exhausting and makes me worse at nearly everything else, from remembering theory to social skills.

However, in a written exam (or at home posting on the Internet), I don't hesitate to block out distracting stimuli and focus on one thing. I can still churn out an essay in 2-3 hours, and I write novels for fun and can focus about as well as I used to be able to, although exhaustion is a confounding factor. (I'm sleep deprived a lot of the time, because of shift work, which wasn't the case when I was in high school). My comfort zone is still absentmindedness, being 'zoned out' and focused on my own thoughts, and I don't think this will ever change–but I already have a degree of situational awareness that I can switch on at will, which will probably increase over my next few years of work experience.