i just tested it on an Azmodan ms8 kill with a timer, and it seems like there is a cap between 2,2 and 2,5 attack speed which might be related to the cast animation.
ive tested azmodan ms8 with 2 different daggers that both do result in the same shown screen dmg, but with different speed.
i get 930-980k dps with 2,5 attack speed
and i get 930-980k dps with 2,3 attack speed
thats a bit strange...
has anyone else experiences or tests done?