Interesting idea. Disclaimer, I don't play rogue.
I looked at our rogue's damage breakdown on Gruul the other night and it was 36.59% Melee, 22.57% SS, and 12.75% Main Gauche. (plus some other stuff that is either super low or not affected by weapon)
I could not figure out how to separate the melee damage into main hand and off hand swings first of all. So I have no idea how that melee number would be affected by switching to dagger. I can make some guesses though... He is rocking a 685 MH and 680 OH. Now a 685 MH Fist weapon has 389dps a 685 Dagger has 389dps too! So it would sound like the melee part of the damage should remain about the same. SS damage is based on MH weapon, right? Fist weapon is like 1k average damage, dagger is like 700 average damage. So 30% less.
So assuming the melee damage stays the same, he would be dropping 23% of his damage by 30%. The speed difference drop from 2.6 to 1.8 is 30%.... So it would seem like if he equipped a dagger in his MH that it would even out the damage. His melee damage would be the same, his SS damage would drop by roughly 30% but his Main Gauche damage would raise by roughly 30%. Of course dropping 23% of your damage by 30% and increasing 13% of your damage by 30% does not add up.
Just guessing I would think you would end up needing so much mastery to make that work that it would be just flat better to have that much master and a normal MH.