yeah, ghostly whisper relies pretty heavily on commando for the bulk of its power.
To add another reason to get it, it's all the crowd control a dark lord really needs against ct bosses. Most of the mobs just melt when they get in ghostly's range while commando is backing it up.
As far as blood testament goes, it adds 15% to the final damage at mastery. I just tested scythe with and without testament on torobbies.
done with 1229 ma and 199% dark, skill used was mastered scythe:
no testament crit: 108,642
testament crit: 124,938