Interesting about people's experiences - I have recently successfully upgraded twice with points only from J to F on the SYD-DFW flight. First time (May 8) I only requested 14 days out and was successful and second time (June 14) was 8 days out and also successful. As a Gold too, so I thought I would have next to no chance (demand and points plus pay option) so was pleasantly surprised twice. Both times cabin was 100% full.
I thought I read somewhere that fare class helps i.e. a flex business ticket is prioritised over a sale business ticket? (both times were flex).