The short answer is that I wrote a spreadsheet to figure it out. Or, at current, a python library. However, to motivate the answer a little bit: LLLL is a 362.8 DPS 1.8 speed dagger; this means the average base weapon damage is 653.5, while an ilvl 346 1.4 speed dagger only does 573.5; hence your instant attacks are significantly smaller as a result. More generally, swapping a 333 1.8 dagger out for a 346 1.4 dagger gives:
- Smaller Backstabs
- Smaller Mutilates
- Fewer poison procs
- Lower proc uptime
In exchange for
- More base stats
- Higher autoattack damage
And when you work out the influence of all those factors, it works out that the 1.8 speed dagger does more total damage.
To be a little more numerical about the comparison: for a particular gear set I've been assessing, MH weapon DPS has an EP value of about 3.3, and using a 1.8 MH instead of a 1.4 gives roughly a 410 EP benefit. Those precise values will fluxuate somewhat based on variations in gear, but its sufficient to give us a ballpark figure. So comparing LLLL to Throat Slasher, we gain 15 agi and 29 rating stats for a total benefit of ~70 EP from stats, as well as 47 DPS for another 155 EP, for a total benefit of 225 EP... which is barely over half the 410 we lose from the speed difference. So a 1.4 weapon that's *one* tier better isn't anywhere close to as good, but a weapon two tiers higher would plausibly work out to be slightly better.