So if a drop isn't 100% guaranteed it increases the base rate by 50/100% (for example using a 100% war rant for an item with a drop rate of 1% raises the chance to 2% and a drop rate of 50% would be raised to 100%) I believe I remember a GM or someone working with Gamigo saying it works on all drops which implied it worked on quest drops that aren't 100% as well and chest/gathering drops (for example the LN egg). It would be nice to have this reconfirmed by Gamigo though as the amount of time that's passed seems to make people more skeptical of this information.
Assuming this came from one of my previous comments, then yes, this would be the case.
As far as I am aware, any form of boost will raise that drop rate % by what ever % you are trying to boost it up by.
So if an Item has a 10% chance of dropping a 100% boost will boost it to 20%, a 50% will boost it to 15% chance of dropping.
Hope this helps!