It’s not that I necessarily “see” extra progress with my supplement plan, but I can feel when it’s off. If I skip the nighttime Mag-10, I get more intense DOMS that sets in sooner (like, tender-to-the-touch muscle soreness starting the day after lifting vs soreness only when I stretch a muscle that sets in 48 hours later). Also, during the session, when I’ve played around with dosing, there’s less pep in my step between sets and at the end of my session, compared to feeling 100% straight through.
I think it’s a safe bet that the majority of competitors keep some type of protein powder in their plan the majority of the time leading up to a show. I’m pretty sure @robstein was including Metabolic Drive up until about one week before his most recent contest.