As a Gold... it is mixed...
Recently, I have flew a lot segments (I think 12-14 in 2 weeks). Here is what I noticed, the Thursday/Sunday flights have the worst chance of upgrades, but mid-week is pretty much a mix. Also, Seattle seems to have some high loads... I was shocked to see how many times the flight goes out packed.
This group of trips... 3 out of the 4 upgradable segments went through.... ^