It means Crucial (Micron) successfully tested the IC at a speed of 400MHz (DDR2-800).
This doesn't necessarily mean the IC won't do higher speeds, proven by the fact that it's used in a PC2-8500 kit.
All PC2-8500 memory is actually overclocked PC2-6400, because the latter is the highest DDR2-specification designed by JEDEC.
No, I don't think it's that simple. Crucial Ballistix PC2-6400 overclocks pretty good and they use the same -25F ICs.
Try running it at 5-5-5-15. Clockspeed is more important than timings and you will need looser timings to gain a higher clockspeed.
Use a higher voltage if necessary, but try not to go over 2.3V for too long. Also put a fan over the modules.
No, that doesn't help. Both Micron D9GMH and D9GKX are 10x10mm.

Bookmarks