Rolling as a warm-up for a comp is great, gets your mind and body in the right place. Running around, skipping etc doesn't do that IMO. But as for avoiding the adrenaline dump? You can't completely, it's your body's natural response to that situation. What you can do though is compete lots so that pre-comp becomes a familiar situation, and roll in your warm-up, so that your match isn't the first bit of BJJ you've done that day.