So, you were getting 20% haste from the frost dk for your testing. If an unholy DK were on the dummy then your (and the frost dk's) damage would have both been inflated (the frost DKs more than yours). You need to keep these things in mind.
granted, again i said 3,5k isn't hard to achieve on your own.
3.9 was indeed benefiting for a snared target and the haste.