My experience with getting preferred seats and airport standbys for an earlier flight has been extremely good with DL as a ST Elite Plus.
OpUps however not. Last autumn I was on a flight MCO-JFK which was overbooked by something like 15 in Y but had many seats available in F (asked at check-in). The screen at the gate showed only about 5 people in line for complimentary upgrades and Y full. Even though I boarded last I was given my original Y seat, and could take a peak at all the leisure travellers in F. Though I can never know for sure, I have a pretty big hunch many of the upgraded people on this route had no ST status.