Over a year later I actually went and tested this, along with a few other things.
See if you can spot the surprising finding! Nevermind, I derped. Fixed now.
http://wiki.bluegartr.com/bg/Ninja_Tool_Expertise
* Testing Yonin now
The two things left to test are:
1) Gosha Sarashi - I won't be camping this NM, as
FFXIDB shows a 5% drop rate and my devotion to FFXI science only goes so far.
2a) Yonin - I might test this tomorrow during the day. The effect probably decays, so I have to interpret any result I get as the average NTE over whatever time period I use.
2b) Yonin with "Enhances Yonin" gear - Should tell us if this equipment enhances Yonin's NTE.