First of all, the default SimCraft priority for Blood DKs is terrible if you want to evaluate DPS. I use my own personal priority for SimCraft because it's actually accurate for near maximum DPS without sacrificing survivability. You can't just use a crappy priority, which is evident by the fact that it doesn't even include Death and Decay, just to skew numbers.
Here's what I use:
# Custom consumables.
# Executed before combat begins. Accepts non-harmful actions only.
# Snapshot raid buffed stats before combat begins and pre-potting is done.
# Executed every time the actor is available.
Second of all, I simmed him in his Fury gear because that's his main spec. Obviously that isn't going to result in 400k simmed DPS because he's not wearing full heroic warforged gear which is what all of the T16H profiles use.
Anyways, you're kind of missing the point which is that it's entirely situational, saying that a weapon upgrade is always better to give to a DPS over a tank is flat out wrong. I'm not going to get into a debate on specific numbers of every single spec that uses 2-handers.