Don't do this much... but since people were asking...
http://lodestone.finalfantasyxiv.com...entry?e=382120
That blog post is saying clear raids under 15min with 5+ chest and it's 100% drop on the mythril needed for the AF weapon.
Shows pic of the person with 4 of them in inventory.
Hope that helps some... I would probably never reach this stage before 2.0 so useless info for me... /sigh
EDIT: the blog has update says 17min TA also confirmed 100% drop. -
EDIT 2: Further reading other people's blog posts it seems it's different loot pool when you do the regular 25min TA it wont give you the 100% guarantee drop from the chest. Hast to be 17 minutes or under. At least this is what I got for AV one....