does it still scale better with slow daggers?

Slower daggers deal more damage. Yes.
However the damage difference is quite small.

Its as simple as this

Slow daggers equals BURST.
Fast daggers equals STEADY damage.

Guess what you want for what? :)

Different daggers will effect your Mutilate dps in different ways according to their stats so there is no perfect fast/fast slow/fast slow/slow answer.

For me a slow main hand (The Flesh Shaper) with decent stats and a fast offand (Librarian's Paper Cutter) is working perfectly.

From spreadsheet testing there are no better DPS increases over my offhand dager until I can get the Omen of Ruin from 10 man naxx (im not in a 25 man raiding guild).

On my server the Librarians Paper Cutter costs about 60g from the AH, and using 2 of them as starter daggers is also viable.

Look at the Elitist Jerks pocket guide for some decent general info http://elitistjerks.com/f78/t37183-pocket_guide_wotlk/.

EDIT: opps, posted on my alt

For PvP 2 slow daggers are better simply because you dont do damage with poisons as you do in PvE, the reason for choosing faster daggers over slower is the procs of instant poisons and focused attacks so the overall damage will be better with faster once.

if you want high arena burst 1,8 daggers is the way to go!