Using lvl 85 weps
http://db.mmo-champion.com/i/55067/ - axe
http://db.mmo-champion.com/i/55068/ - dagger
Axe dmg (w/o AP)
(462.5*2.6)*1.9 +700 = 2984.75 non crit
Dagger dmg (w/o AP)
(462.5*1.8)*2.74+1012 = 3293.05 non crit
Difference : 308.3
With AP adding 400 dps (can't remember AP values)
Axe : ((462.5+400)*2.6)*1.9+700=4960.75
Dagger: ((462.5+400)*1.8)*2.74+1012=5265.85
Difference : 305.1
To calculate the amount of +dps we need in order for 2.6 speed wep to be > 1.8 speed dagger we can use
(X)*2.6*1.9+700=x*2.74*1.8+1012
X*4.94+700=X*4.932+1012
0.068X=312
X= 4588.23529411765 dps , aka we're not getting this any time soon.
If we deside to look at the same model but with 2.7 speed non dagger we get
Non AP : 3072.625
+400 dps AP : 5124.625
DPS at which 2.7 speed wep = 1.8 speed dagger : (0.198 dmg per each dps) 1575.75757575758 which is a) a lot closer, b) probably achievable in 2nd-3rd tier of cata (granted we get access to 2.7 speed phys dps wep)
Also, it's sami safe to assume, that the 3rd version of barmen shanker (aka brewfest dagger) adjusted to lvl 85, will blow most weps out of water based on ambush dmg (2.0 speed ftw)