Figure I'd put it in raids & dungeons since it's more relevant to have altering bis lists for well... raiders.
Anyways one thing I've noticed is that as you all know a gem socket takes item budget. Except when you upgrade an item the secondary / primary stats are increased by a % instead of a strict number.
Say one item lvl upgrade of 4 increases every stat by 4%
If you have a socket that takes up 20/100 item budget you will now have a 4% increase to 80/100 opposed to 4% of 100/100.
These numbers are very small when it comes to single items but in some cases I've been noticing for myself that extra 20~ secondary stats is enough to make a 2nd bis item full on bis.