During the test I used these stats
Illusion 16
Dagger 12
1)First, about dagger mastery. The little 2% to do dual attack: many mesmers wondered about it stacks with IW. At 12 of dagguer, you have 24% chance to make a dual att.
Question: Did IW stack with Dual att from Dagguer Mastery?
My answer: Yes
When you're in IW, you can do a dual attack and do another 42 dmg to your foe.
2)Speed. Some says daggers go faster than swords, axe and others say go the same speed. Which say the truth?
Question: Did the daggers go faster than swords and axe?
My answer:Yes... again
Sword and Axe go about 1.33/sec and Daggers, I calculate about 1.29/sec. So dagguers are faster.
3)Effectivness, the REAL question come soon. Everybody knows about Me/W (Flurry) or Me/R (Beastial Fury A.K.A Tiger Fury) to increase speed and deal more dmg.
Question: Is it effective if we compare the others (Me/W and Me/R)?
My answer: Yes and no.
For No, "Quicker you kill your enemies, the better" I can say with IW. Because with Flurry/Fury, you can pack more dmg than daggers and, from another way, you dont have any focus with you when you use it.
For yes, If you dont care about speed, then you have, at least, one free slot to put a skill of your choice. Generally, when you play IW, you try to avoid taken dmg from any source. So thats why Lyssa gives us Distortion

It's all I can say for now on IW with Me/A. If you want to try it, go for it. Good luck in your build and good game all. Watch out to dont get stripped by Necromancer or another Mesmer.
If you have questions, post your thread here or leave a comment about it.
-Malin-