Each time I've been marked as SFU (three times in the last 2 years) - I've been bumped up. But each time I've been marked as SFU - the flight was already oversold. Twice I knew by the time I got to the lounge, the third time didn't take effect until the last minute when there were delays on other flights.

For reference on how those occurred - it was always the checkin agent doing it after a friendly conversation where I might have commented on how full the flight is or some other such 'I wonder' type comment without actually asking for an upgrade or SFU - but that tactic only seems to work on rare occasions.

However, on those occasions I was silver and since gold I haven't been SFU-ed and my op-up rate is higher generally anyway.