My current playstyle has me playing Agi-gem Combat and Subtlety. With my Subtlety specs, I play with 4k Crit, then Haste>Mastery>Crit.
Right now I have 3 choices for weapons: my heroic Norushen's Shortblade, a normal Softfoot's Last Resort, and a normal Nazgrim's Gutripper.
Min-maxing in Shadowcraft to the way I like my Subtlety spec, in my current gear, the 2.6 offhand spec comes out about 3k less than the 1.8 offhand spec.
Math time!:
1 - (offhand 2.6 value / offhand 1.8 value)
1 - (336,229 / 339,147.9) = 0.0086