A quick look on wowhead shows almsot all daggers at 1.8 speed, a couple of lower ilevel ones at 1.7 speed, and spellpower ones at 1.6 speed.
They did away with the offhand faster dagger malarky in the pre-mop patch I think.
This. The 1.4 speed offhand became less important when they made it so poisons applied to both weapons, instead of one individual weapon, along with the combining of Deadly and Instant Poison. Combat Potency was also normalized a bit, and made to proc from Combats 'extra swing' mastery.
You are correct, OP, that a 1.4 (or 0.1) offhand would be ideal for sub and mutilate, and likely edge out slow offhands for combat as well. But as has been stated, they changed all of them to 1.8 because, honestly, our gearing was absurd. To be a fully geared rogue back in the day you wanted way too much garbage, and often a tier would have one 1.4 dagger and one 1.8 dagger and each one had exactly one use- be a rogue's mainhand or offhand.
OP - here is your final answer. To prevent clutter and confusion, closing this thread.