WS changes would be fine and good, but Empyreans also need a minor base damage boost (relative to Relics and Mythics). SE just messed up when they released the last set of values, really. It's possible that they were still considering Empyrean WS access to be one of the selling points of the weapons, but the quest to unlock those WSs pretty much took care of that.

Example:
* Twashtar has D89, 176 delay, DEX+20, and using Rudra's Storm gives a 30-50% Double damage aftermath (tends towards the 30%).
* Mandau has D91, 176 delay, Attack+40, Additional Effect: Poison, and gives access to Mercy Stroke (with damage+40%), and 13% Triple damage on the first swing with that hand (about the same as 15% double damage with current gear). Mercy Stroke gives a +5% Critical hit rate (including both hands and WS, iirc) aftermath.

So Mandau is basically a stronger half-AM Twashtar that doesn't rely on aftermath and gives access to Mercy Stroke. Twashtar's Aftermath effect is stronger than Mandau's but Mandau's isn't worthless either ('specially considering Evisceration) and Mercy Stroke is pretty cool and beats Rudra's Storm (which Mandau users also have access to) when stacked at 1000TP.