It might just be because of my preferred subfaction (Host of the Everchosen) but I've never had the problems others have had with killing models. In fact my issue has always been I'm too heavy into killing, and my only loss as of now has been to Tzeentch board control as I simply couldn't compete with 300+ horrors that sat on every obj.
all of my units kill most things I send them at; Knights will average 16 damage on a 4+ save unit which is almost always enough (when they actually get to swing, I tend to use them to not let my Chaos Lord get killed) he averages only 15 damage, but in reality he often does 10 just with his axe(rune blade to give it -3 rend makes it kill most things, and even after that he has the rest of his attacks). And thats all with only guaranteed buffs (rr1s to hit and wound, +1 to wound from Archaon) and is very hard to kill with his 3+ save and d3 healing every turn.
And Archaon obviously kills anything he touches and almost never dies with full reroll saves (again guaranteed) If we add in Daemonic Power from the sorc which isnt very hard to get off honestly, one of the units is pumped up even further.
regardless, I think you can defiantly kill very effectively as StD if you spec for it.
EDIT: I also think that once the Mindstealer comes out StD armies will become much better; effectively old Slaanesh fight last for 100pts is very very good and will help us lock down a dangerous unit and dogpile it.