I'm not 100% sure if the Assassins have their own IAS breakpoints for Whirlwind or if the follow the Barb's tables and assume 1 handed weapons.
I'll be researching that on Thursday AND post a FULL guide on how to make a smitingly good WWsin (in an attempt to regain some face after blckshdwdragon beat the tar out of me for my last Assassin build).