Conventional Wisdom is that the upgrades that are driven through the departure management system (standby upgrades, elite upgrades, etc.), which are computer-processed based on priority/fare/elite status/checkin time, etc. are "safe" regardless where you are (PC, gate, etc.). As long as you've checked in.

However, "battlefield" OpUps may be a different story ... if it's a full flight in Y and you sense that there might be opportunities for OpUps, I think you're best off being at the gate early, perhaps making a courtesy check with the GA to see what loads are like in front and in the rear, is best. I don't believe in giving "The Speech" as any good GA will see right through this.